مادة مظلمة

(تم التحويل من Dark matter)
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ما هي المادة المظلمة؟ وكيف تتشكل؟
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التوزيع المقترح للمادة والطاقة في الكون، اليوم (في الأعلى)، وعند تحرر إشعاع الخلفية الميكروني الكوني (في الأسفل).

في علم الفلك وعلم الكون، المادة المظلمة (dark matter)، هي شكل افتراضي وغير مرئي من المادة لا يتفاعل مع الضوء أو الأشعة الكهرومغناطيسي. وتُستدل على وجود المادة المظلمة من خلال تأثيرات الجاذبية التي لا يمكن تفسيرها بنظرية النسبية العامة إلا إذا كانت كمية المادة الموجودة أكبر مما يمكن رصده. وتحدث هذه التأثيرات في سياق تشكل المجرات وتطورها،[1] عدسة الجاذبية،[2] البنية الحالية للكون المرصود، وموقع الكتلة في تصادمات المجرات،[3] حركة المجرات داخل عناقيد المجرات، وإشعاع الخلفية الصغروي الكوني. يُعتقد أن المادة المظلمة تعمل كدعامات جاذبية للبُنى الكونية.[4] بعد الانفجار العظيم، تجمعت المادة المظلمة في كتل على امتداد خيوط ضيقة، وشكلت عناقيد المجرات العملاقة شبكة كونية على نطاقات تبدو فيها المجرات بأكملها كجسيمات صغيرة.[5][6]

في نموذج لامبدا-سي‌دي‌إم المعياري، فإن محتوى الكتلة-الطاقة للكون هو 5% مادة عادية، و26.8% مادة مظلمة، و68.2% شكل من أشكال الطاقة يُعرف باسم الطاقة المظلمة.[7][8][9][10] وبالتالي، تشكل المادة المظلمة 85% من الكتلة الإجمالية، بينما تشكل الطاقة المظلمة والمادة المظلمة 95% من إجمالي محتوى الكتلة-الطاقة.[11][12][13][14] على الرغم من أن كثافة المادة المظلمة كبيرة في الهالة المحيطة بالمجرة، إلا أن كثافتها المحلية في المجموعة الشمسية أقل بكثير من كثافة المادة العادية. ويبلغ مجموع كتلة المادة المظلمة حتى مدار نپتون حوالي 1017  كجم، أي ما يعادل كتلة كويكب كبير.[15]

لا يُعرف أن المادة المظلمة تتفاعل مع المادة الباريونية العادية والإشعاع إلا من خلال الجاذبية، مما يجعل اكتشافها في المعمل صعباً. التفسير الأكثر شيوعاً هو أن المادة المظلمة عبارة عن جسيم دون ذري لم يُكتشف بعد، مثل الجسيمات الضخمة ضعيفة التفاعل (WIMPs) أو الأكسيونات.[16] الاحتمال الرئيسي الآخر هو أن المادة المظلمة تتكون من ثقوب سوداء بدائية.[17][18][19]

تُصنّف المادة المظلمة إلى "باردة" أو "دافئة" أو "ساخنة" وفقاً لسرعتها (أو تحديداً، طول مسارها الحر). وقد رجّحت النماذج الحديثة سيناريو المادة المظلمة الباردة، حيث تنشأ البُنى من خلال التراكم التدريجي للجسيمات.

على الرغم من أن مجتمع الفيزياء الفلكية يقبل عموماً وجود المادة المظلمة،[20] يدعو عدد قليل من علماء الفيزياء الفلكية، الذين أثارت اهتمامهم ملاحظات محددة لا يُفسرها مفهوم المادة المظلمة التقليدية تفسيراً وافياً، إلى تعديلات مختلفة على قوانين النسبية العامة. تشمل هذه التعديلات الديناميكا النيوتونية المعدلة، والجاذبية الموترية-المتجهية-القياسية، والجاذبية الإنتروپية. حتى الآن، لا تستطيع أي من نظريات الجاذبية المعدلة المقترحة تفسير جميع الأدلة الرصدية في آن واحد، مما يشير إلى أنه حتى في حال تعديل الجاذبية، سيظل وجود شكل من أشكال المادة المظلمة ضرورياً.[21]


التاريخ

التاريخ المبكر

تتمتع فرضية المادة المظلمة بتاريخ مفصل.[22][23] ناقش وليام طومسون، لورد كلڤن، العدد المحتمل للنجوم حول الشمس في ملاحق كتاب استند إلى سلسلة من المحاضرات التي ألقاها عام 1884 في بالتيمور.[24][22] استنتج كلڤن كثافتها باستخدام تشتت السرعة المرصود للنجوم القريبة من الشمس، بافتراض أن عمر الشمس يتراوح بين 20 و100 مليون سنة. وتساءل عما سيحدث لو كان هناك ألف مليون نجم ضمن مسافة 1 كيلو فرسخ فلكي من الشمس (حيث يكون اختلاف المنظر عند هذه المسافة 1 [[دقيقة وثانية القوس|ملي-ثانية قوس). وخلص كلڤن إلى أن:

قد تكون العديد من نجومنا المفترضة التي يبلغ عددها آلاف الملايين - وربما غالبيتها العظمى - أجراماً مظلمة.[24][25]

عام 1906، استخدم هنري پوانكاريه[26] المصطلح الفرنسي [matière obscure] ("المادة المظلمة") في نقاشه لعمل كلڤن.[26][25] لقد وجد أن كمية المادة المظلمة يجب أن تكون أقل من كمية المادة المرئية، لكن اتضح أن هذا الاكتشاف خاطئ.[25][22]

ثاني من اقترح وجود المادة المظلمة باستخدام سرعات النجوم كان الفلكي الهولندي ياكوبوس كاپتين عام 1922.[27][28]

تشير إحدى منشورات عالم الفلك السويدي نوت لوندمارك عام 1930 إلى أنه كان أول من أدرك أن الكون يجب أن يحتوي على كتلة أكبر بكثير مما يمكن رصده.[29] افترض رائد علم الفلك الراديوي الهولندي يان أورت أيضاً وجود المادة المظلمة عام 1932.[28][30][31] كان أورت يدرس حركات النجوم في الجوار المجري ووجد أن الكتلة في مستوى المجرة يجب أن تكون أكبر مما رُصد، لكن تبين لاحقاً أن هذا القياس غير صحيح.[32]

عام 1933، درس عالم الفيزياء الفلكية السويسري فريتس تسڤيكي عناقيد المجرات أثناء عمله في معهد كاليفورنيا للتكنولوجيا وتوصل إلى استنتاج مماثل.[33][أ][34] طبّق تسڤيكي نظرية ڤيريال على عنقود الذؤابة، وحصل على دليل على وجود كتلة غير مرئية أطلق عليها اسم dunkle Materie ('المادة المظلمة'). قدّر تسڤيكي كتلة العنقود بناءً على حركة المجرات القريبة من حافته، وقارن ذلك بتقدير آخر مبني على سطوعه وعدد مجراته. وقدّر أن كتلة العنقود تزيد بنحو 400 ضعف عن الكتلة المرئية. كان تأثير جاذبية المجرات المرئية ضئيلاً للغاية بالنسبة لهذه المدارات السريعة، مما يعني أن الكتلة مخفية عن الأنظار. بناءً على هذه الاستنتاجات، استنتج تسڤيكي أن بعض المادة غير المرئية هي التي وفرت الكتلة وقوة الجذب المرتبطة بها للحفاظ على تماسك العنقود.[35] كانت تقديرات تسڤيكي خاطئة بأكثر من قيمة أسية، ويرجع ذلك أساساً إلى قيمة ثابت هبل المهملة؛[36] تُظهر الحسابات نفسها اليوم نسبةً أقل، باستخدام قيم أكبر للكتلة اللامعة. ومع ذلك، فقد استنتج تسڤيكي بشكل صحيح من حساباته أن معظم المادة الجاذبية الموجودة كانت مظلمة.[25] لكن على عكس النظريات الحديثة، اعتبر تسڤيكي "المادة المظلمة" مادة عادية غير لامعة.[22]:III.A

ظهرت مؤشرات إضافية على وجود شذوذ في نسبة الكتلة إلى الضوء من خلال قياسات منحنيات دوران المجرات. عام 1939، نشر هوراس بابكوك منحنى دوران مجرة المرأة المسلسلة، والذي أشار إلى أن نسبة الكتلة إلى اللمعان تزداد شعاعياً.[37] عزا ذلك إما إلى امتصاص الضوء داخل المجرة أو إلى تغيرات في ديناميكيات الأجزاء الخارجية من المجرة الحلزونية، بدلاً من وجود مادة غير مرئية. بعد تقرير بابكوك عام 1939 عن دوران سريع غير متوقع في أطراف مجرة ​​المرأة المسلسلة ونسبة الكتلة إلى الضوء تبلغ 50؛ عام 1940، اكتشف أورت وكتب عن الهالة الكبيرة غير المرئية لمجرة NGC 3115.[38]

السبعينيات

ترسخت فرضية المادة المظلمة بشكل كبير في السبعينيات. جُمعت عدة ملاحظات مختلفة للتأكيد على أن المجرات يجب أن تكون محاطة بهالات من مادة غير مرئية. في ورقتين بحثيتين نُشرتا عام 1974، توصلت مجموعتان بحثيتان مستقلتان إلى هذا الاستنتاج: الأولى في پرنستون، نيوجرزي، بواسطة جرميا أوسترايكر، جيم پيبلز، وعاموس ياهيل، والثانية في تارتو، إستونيا، بواسطة يان إيناستو، إن ساريت، وأنتس كاسيك.[39]

كان شكل منحنيات دوران المجرة أحد الملاحظات التي قدمت دليلاً على وجود هالات مجرية من المادة المظلمة. أُجريت هذه الملاحظات في علم الفلك البصري والراديو. في علم الفلك البصري، عملت ڤيرا روبن وكنت فورد باستخدام مطياف جديد لقياس منحنى سرعة المجرات الحلزونية التي تُرى من جانبها بدقة أكبر.[40][41][42]

في الوقت نفسه، كان علماء الفلك الراديوي يستخدمون تلسكوبات راديوية جديدة لرسم خريطة الخط 21 سم للهيدروجين الذري في المجرات القريبة. غالباً ما يمتد التوزيع القطري للهيدروجين الذري بين النجوم (المنطقة H I) إلى مسافات مجرية أكبر بكثير مما يمكن رصده كضوء نجمي جماعي، مما يوسع نطاق المسافات التي يمكن أخذ عينات منها لمنحنيات الدوران - وبالتالي توزيع الكتلة الكلية - إلى نظام ديناميكي جديد. رُسمت خريطة مبكرة لمجرة المرأة المسلسلة باستخدام تلسكوب يبلغ قطره 91 متراً في گرين بانك[43] وطبق يبلغ قطره 76 متراً في جودرل بانك[44] وظهر بالفعل أن منحنى دوران HI لم يتتبع الانخفاض المتوقع من المدارات الكپلرية.

مع توفر أجهزة استقبال أكثر حساسية، Roberts & Whitehurst (1975)[45] تمكن الباحثون من تحديد سرعة دوران مجرة ​​المرأة المسلسلة حتى مسافة 30 ك.ف.ف.، وهو ما يتجاوز بكثير نطاق القياسات البصرية. وهذا يوضح ميزة تتبع قرص الغاز عند أنصاف أقطار كبيرة؛ كما هو موضح في "الشكل 16" من تلك الورقة البحثية[45] يجمع البيانات البصرية[42] (مجموعة النقاط عند أنصاف أقطار أقل من 15 كيلوبارسيك مع نقطة واحدة أبعد) مع بيانات HI بين 20 و30 ك.ف.ف، مما يُظهر استواء منحنى دوران المجرة الخارجية؛ المنحنى المتصل الذي يبلغ ذروته في المركز هو كثافة السطح البصرية، بينما يُظهر المنحنى الآخر الكتلة التراكمية، التي لا تزال ترتفع خطيًا عند القياس الأبعد. بالتوازي مع ذلك، كان يجري تطوير استخدام مصفوفات التداخل الطيفي لـ HI خارج المجرة. نشر شوستاك وروجستاد[46] منحنيات دوران HI لخمسة حلزونات رُسمت باستخدام مقياس التداخل في وادي أوينز؛ وكانت منحنيات دوران جميعها مسطحة للغاية، مما يشير إلى قيم كبيرة جداً لنسبة الكتلة إلى الضوء في الأجزاء الخارجية من أقراص HI.[46]

عام 1978، قدم ألبرت بوسما مزيداً من الأدلة على منحنيات الدوران المسطحة باستخدام بيانات من تلسكوب وستربورك الراديوي التخليقي.[47]

عام 1978، قدم ستيگمان وزملائه[48] دراسة وسّعت حسابات الكثافة المتبقية الكونية السابقة لتشمل أي لپتون افتراضي مستقر، متعادل كهربائياً، وذو طاقة ضعيفة، موضحة كيف ستتلاشى وفرة هذا الجسيم في الكون المبكر، ومقدمةً تعابير تحليلية تربط كتلته ومقطع التفاعل الضعيف بكثافة المادة الحالية. ومن خلال فصل التحليل عن خصائص النيوترينو المحددة ومعالجة المرشح بشكل عام، وضع المؤلفون إطاراً أصبح فيما بعد النموذج القياسي للجسيمات الضخمة ضعيفة التفاعل (WIMPs)[49] ولمقارنة نماذج فيزياء الجسيمات بالقيود الكونية. على الرغم من أن الأعمال اللاحقة قد حسّنت المنهجية واستكشفت العديد من المرشحين البديلين، إلا أن هذه الورقة البحثية مثّلت أول معالجة صريحة ومنهجية للمادة المظلمة كنوع جديد من الجسيمات يتجاوز النموذج المعياري.[50]

بحلول أواخر السبعينيات، أصبح وجود هالات المادة المظلمة حول المجرات حقيقة معترف بها على نطاق واسع، وأصبح مشكلة رئيسية لم تُحل في علم الفلك.[39]

1980–1990s

A stream of observations in the 1980–1990s supported the presence of dark matter. Persic, Salucci & Stel (1996) is notable for the investigation of 967 spirals.[51] The evidence for dark matter also included gravitational lensing of background objects by galaxy clusters,[52]:{{{1}}} the temperature distribution of hot gas in galaxies and clusters, and the pattern of anisotropies in the cosmic microwave background.

According to the current consensus among cosmologists, dark matter is composed primarily of some type of not-yet-characterized subatomic particle.[53][54] The search for this particle, by a variety of means, is one of the major efforts in particle physics.[55]


التعريف التقني

انظر أيضاً: Friedmann equations

In standard cosmological calculations, "matter" means any constituent of the universe whose energy density scales with the inverse cube of the scale factor, i.e., ρ ∝ a−3 . This is in contrast to "radiation", which scales as the inverse fourth power of the scale factor ρ ∝ a−4 , and a cosmological constant, which does not change with respect to a (ρ ∝ a0).[56] The different scaling factors for matter and radiation are a consequence of radiation redshift. For example, after doubling the diameter of the observable Universe via cosmic expansion, the scale, a, has doubled. The energy of the cosmic microwave background radiation has been halved (because the wavelength of each photon has doubled);[57] the energy of ultra-relativistic particles, such as early-era standard-model neutrinos, is similarly halved.[ب] The cosmological constant, as an intrinsic property of space, has a constant energy density regardless of the volume under consideration.[56]

In principle, "dark matter" means all components of the universe which are not visible but still obey ρ ∝ a−3 . In practice, the term "dark matter" is often used to mean only the non-baryonic component of dark matter, i.e., excluding "missing baryons".[58] Context will usually indicate which meaning is intended.

دور المادة المظلمة

لعبت المادة المظلمة دوراً أساسياً في تخليق النجوم في البدايات الأولى من الكون، إذا كانت المادة المظلمة على هذه الحالة، على أية حال، يجب أن يشتمل المادة المظلمة على الجزيئات المعروفة بـ"النيوترونات العقيمة". قام بيتر بيرمان من معهد ماكس بلانك لعلوم الفلك الإشعاعي في بون، وألكسندر كوسينكو، من جامعة كاليفورنيا في لوس أنجليس، باظهار أنه عندما تضمحل النيوترونات العقيمة، فإنها تسرّع عملية خلق جزيئات الهيدروجين، هذه العملية ساعدت على إضاءة النجوم الأولى فقط منذ حوالي 20 إلى 100 مليار سنة بعد الانفجار الكبير، كل هذه المعطيات تعطينا تفسيراً بسيطاً لبعض الملاحظات المحيرة الأخرى التي تتعلق بالمادة المظلمة، النيوترونات العقيمة، والمادة المضادة.

اكتشف العلماء بأن تلك النيوترونات لها كتلة خلال تجارب قياس ذبذبة النيوترونات. هذا قاد إلى افتراضات بإن النيوترونات العقيمة الموجودة - هي كذلك معروفة أيضا بالنيوترونات اليمينية، وبأنها لا تشارك في التفاعلات الضعيفة مباشرة، ولكنها تتفاعل من خلال خلطها مع النيوترونات العادية.إن العدد الكلي للنيوترونات العقيمة غير واضح، إذا كانت كتلة المادة المظلمة نعادل بضعة كيلو إلكترونوفولتز (1 KeV تعادل مليون كتلة ذرة الهيدروجين)، فإنها توضح ضخامة الكتلة المفقودة في الكون، أحيانا، تسمى المادة المظلمة، ودعمت ملاحظات الفلكيين الفيزيائيين وجهة نظر باحتمال بأن المادة المظلمة تشتمل على النيوترونات العقيمة.

المادة المظلمة الباريونية والغير باريونية

تقدم مشاهدات فيرمي للمجرات القزمة رؤية جديدة للمادة المظلمة.

من الممكن لنسبة صغيرة من المادة المظلمة أن تكون مادة مظلمة باريونية: الأجسام الفلكية مثل هالة الأجسام الثقيلة المدمجة المؤلفة من مادة عادية تبعث القليل أو لاتبعث على الإطلاق أي اشعاع كهرومغناطيسي، ويشير التوافق مع المشاهدات الأخرى إلى عدم إمكانية الغالبية العظمى من المادة المظلمة الموجودة في الكون لأن تكون مادة مظلمة باريونية أي أنها غير مشكلة من الذرات ولا يمكنها التآثر مع المادة العادية عبر القوى الكهرو مغناطيسية ولا تحمل جسيماتها أي شحنة كهربائية، وتتضمن المادة المظلمة الغير باريونية النيوترينوهات مع إمكانية وجود جسيمات افتراضية مثل الأكسيومز أو الجسيمات فائقة التناظر، وعلى عكس المادة المظلمة الباريونية، لاتساهم المادة المظلمة غير الباريونية في تشكيل العناصر في بداية الكون "الاصطناع النووي للانفجار العظيم" وبالتالي يتم الكشف عن وجودها فقط من خلال تجاذبها الثقالي، بالإضافة إلى ذلك، لو كانت الجسيمات المؤلفة للمادة المظلمة غير الباريونية فائقة التناظر فإنها من الممكن أن تخضع لتفاعلات الإفناء مع نفسها مما يؤدي إلى ملاحظتها من النواتج الفرعية مثل الفوتونات والنيوترينوهات "كشف غير مباشر".

تصنف المادة المظلمة غير الباريونية من حيث كتلة الجسيمات المفترضة لتشكيلها و/أو السرعة النموذجية لانتشار تلك الجسيمات (حيث أن الجسيمات الأثقل تكون أبطأ). هناك ثلاثة افتراضات بارزة للمادة المظلمة الغير باريونية وهي المادة المظلمة الساخنة والدافئة والباردة مع إمكانية المزج بينهم. النموذج الأكثر مناقشة للمادة المظلمة الغير باريونية مبني على فرض المادة المظلمة الباردة ويفترض الجسيم المرتبط به ليكون في الغالب جسيم ثقيل ضعيف التآثر. من الممكن للمادة المظلمة الساخنة أن تتألف من النيوترينوهات الثقيلة. تؤدي المادة المظلمة الباردة إلى تشكيل "قاعدي-علوي" لبنية الكون والمادة المظلمة الساخنة إلى تشكيل "قمي-سفلي" لنفس البنية.

نظرية بيرمان وكوسينكو

.

Dunkle Materie: التعارض بين سرعة دوران النجوم في مجرتناالمشاهدة(احمر) بالمقارنة بالحسابات المبنية على كمية المادة المنظورة فيها (أزرق)، مما يدعو طبقا لقوانين الجاذبية إلى الاعتقاد بوجود كتلة جسيمات أو أجسام غير منظورة تساعد في عملية الجذب، وسميت المادة المظلمة.

تسلط نظرية بيرمان وكوسينكو الضوء على عدد من الألغاز الفلكية الغير مفسرة، أولا، وأثناء الانفجار الكبير، كانت كتلة النيوترونات المخلوقة في الانفجار الكبير تساوي ما نحتاجه لتفسير المادة المظلمة، ثانياً، هذه الجسيمات يمكن أن تكون الحل لمشكلة كبيرة حول لماذا تتحرك البولسرات بسرعة كبيرة.

البولسرات هي نجوم نيوترونية تدور بسرعة عالية جداً، ونشأت نتيجة لانفجار مستعر فائق (سوبرنوفا) وتكون عادة مقذوفة في اتجاه واحد. الانفجار أعطاها " دفعاً قويا"، مثل محرك صاروخ. مما يجعل البولسرات تسير بسرعات كبيرة تصل إلى مئات الكيلومترات في الثانية - وأحياناً إلى الآلاف. مصدر هذه السرعات تبقى مجهولة، لكن إشعاع النيوترونات العقيمة توضح تحركات البولسرات.

يحتوي سديم القيثارة على بولسرات سريعة جداً، إذا كانت المادة المظلمة صنعت جزيئات مؤينة في الكون - كما يقترح بيرمان وكوسينكو- بأن حركة البولسرات هي التي أنشأت سديم القيثارة.

كما ان النيوترونات العقيمة يمكن أن تساعد على توضيح انعدام المادة المضادة في الكون. في بدايات الكون الأولى، كانت النيوترونات العقيمة "تسرق" ما يعرف بــ" ترقيمات ليبتون" من البلازما. وفي وقت لاحق، أدت قلة ترقيمات ليبتون إلى تحويلها إلى عدد غير صفري من الترقيمات البريونية. اللاتناظر الناتج بين البريونات (مثل البروتون) والبريونات المضادة (مثل البروتون المضاد) يمكن أن يكون السبب حول عدم وجود مادة مضادة في الكون.

فتشكيلة الثقوب السوداء المركزية في المجرة، بالإضافة إلى التركيب القياسي للمجرات الفرعية، تعطي تفسيراً مفضلا حول النيوترونات العقيمة في المادة المظلمة. والإجماع على آراء متعددة معقدة يقود إلى اتجاه واحد باعتقاد أن المادة المظلمة، في الحقيقة، هي نيوترونات عقيمة[59].

أدلة رصدية

رسم فني يوضح التوزيع المتوقع للمادة المظلمة في مجرة درب التبانة وتظهر على هيئة هالة زرقاء للمادة المحيطة المجرة.[60]

منحنيات دوران المجرة

المقالة الرئيسية: منحنى دوران المجرة
ملف:Comparison of rotating disc galaxies in the distant Universe and the present day.webm
Animation of rotating disc galaxies. Dark matter – shown in red – is more concentrated near the center and it rotates more rapidly.

The arms of spiral galaxies rotate around their galactic center. The luminous mass density of a spiral galaxy decreases as one goes from the center to the outskirts. If luminous mass were all the matter, then the galaxy can be modelled as a point mass in the centre and test masses orbiting around it, similar to the Solar System.[ت] From Kepler's Third Law, it is expected that the rotation velocities will decrease with distance from the center, similar to the Solar System. This is not observed.[61] Instead, the galaxy rotation curve remains flat or even increases as distance from the center increases.

If Kepler's laws are correct, then the obvious way to resolve this discrepancy is to conclude the mass distribution in spiral galaxies is not similar to that of the Solar System. In particular, there may be a lot of non-luminous matter (dark matter) in the outskirts of the galaxy.

تبدد سرعة المجرات

المقالة الرئيسية: Velocity dispersion

Stars in bound systems must obey the virial theorem. The theorem, together with the measured velocity distribution, can be used to measure the mass distribution in a bound system, such as elliptical galaxies or globular clusters. With some exceptions, velocity dispersion estimates of elliptical galaxies[62] do not match the predicted velocity dispersion from the observed mass distribution, even assuming complicated distributions of stellar orbits.[63]

As with galaxy rotation curves, the obvious way to resolve the discrepancy is to postulate the existence of non-luminous matter.

عناقيد المجرات

Galaxy clusters are particularly important for dark matter studies since their masses can be estimated in three independent ways:

  • From the scatter in radial velocities of the galaxies within clusters
  • From X-rays emitted by hot gas in the clusters. From the X-ray energy spectrum and flux, the gas temperature and density can be estimated, hence giving the pressure; assuming pressure and gravity balance determines the cluster's mass profile.
  • Gravitational lensing (usually of more distant galaxies) can measure cluster masses without relying on observations of dynamics (e.g., velocity).

Generally, these three methods are in reasonable agreement that dark matter outweighs visible matter by approximately 5 to 1.[64]


The Bullet Cluster: HST image with overlays. The total projected mass distribution reconstructed from strong and weak gravitational lensing is shown in blue, while the X-ray emitting hot gas observed with Chandra is shown in red.


Bullet Cluster

المقالة الرئيسية: Bullet Cluster

The Bullet Cluster is the result of a recent collision of two galaxy clusters. It is of particular note because the location of the center of mass as measured by gravitational lensing is different from the location of the center of mass of visible matter. This is difficult for modified gravity theories, which generally predict lensing around visible matter, to explain.[65][66][67][68] Standard dark matter theory however has no issue: the hot, visible gas in each cluster would be cooled and slowed down by electromagnetic interactions, while dark matter (which does not interact electromagnetically) would not. This leads to the dark matter separating from the visible gas, producing the separate lensing peak as observed.[69]

Gravitational lensing

One of the consequences of general relativity is the gravitational lens. Gravitational lensing occurs when massive objects between a source of light and the observer act as a lens to bend light from this source. Lensing does not depend on the properties of the mass; it only requires there to be a mass. The more massive an object, the more lensing is observed. An example is a cluster of galaxies lying between a more distant source such as a quasar and an observer. In this case, the galaxy cluster will lens the quasar.

Strong lensing is the observed distortion of background galaxies into arcs when their light passes through such a gravitational lens. It has been observed around many distant clusters including Abell 1689.[70] By measuring the distortion geometry, the mass of the intervening cluster can be obtained. In the weak regime, lensing does not distort background galaxies into arcs, causing minute distortions instead. By examining the apparent shear deformation of the adjacent background galaxies, the mean distribution of dark matter can be characterized. The measured mass-to-light ratios correspond to dark matter densities predicted by other large-scale structure measurements.[71][72]

Type Ia supernova distance measurements

المقالة الرئيسية: Type Ia supernova and Shape of the universe

Type Ia supernovae can be used as standard candles to measure extragalactic distances, which can in turn be used to measure how fast the universe has expanded in the past.[73] Data indicates the universe is expanding at an accelerating rate, the cause of which is usually ascribed to dark energy.[74] Since observations indicate the universe is almost flat,[75][76][77] it is expected the total energy density of everything in the universe should sum to 1 (Ωtot ≈ 1). The measured dark energy density is ΩΛ ≈ 0.690; the observed ordinary (baryonic) matter energy density is Ωb ≈ 0.0482 and the energy density of radiation is negligible. This leaves a missing Ωdm ≈ 0.258 which nonetheless behaves like matter (see technical definition section above) – dark matter.[78]

Redshift-space distortions

Large galaxy redshift surveys may be used to make a three-dimensional map of the galaxy distribution. These maps are slightly distorted because distances are estimated from observed redshifts; the redshift contains a contribution from the galaxy's so-called peculiar velocity in addition to the dominant Hubble expansion term. On average, superclusters are expanding more slowly than the cosmic mean due to their gravity, while voids are expanding faster than average. In a redshift map, galaxies in front of a supercluster have excess radial velocities towards it and have redshifts slightly higher than their distance would imply, while galaxies behind the supercluster have redshifts slightly low for their distance. This effect causes superclusters to appear squashed in the radial direction, and likewise voids are stretched. Their angular positions are unaffected. This effect is not detectable for any one structure since the true shape is not known, but can be measured by averaging over many structures. It was predicted quantitatively by Nick Kaiser in 1987, and first decisively measured in 2001 by the 2dF Galaxy Redshift Survey.[79] Results are in agreement with the Lambda-CDM model.

Lyman-alpha forest

المقالة الرئيسية: Lyman-alpha forest

In astronomical spectroscopy, the Lyman-alpha forest is the sum of the absorption lines arising from the Lyman-alpha transition of neutral hydrogen in the spectra of distant galaxies and quasars. Lyman-alpha forest observations can also constrain cosmological models.[80] These constraints agree with those obtained from WMAP data.



Cosmic microwave background

المقالة الرئيسية: Cosmic microwave background

Although both dark matter and ordinary matter are matter, they do not behave in the same way. In particular, in the early universe, ordinary matter was ionized and interacted strongly with radiation via Thomson scattering. Dark matter does not interact directly with radiation, but it does affect the cosmic microwave background (CMB) by its gravitational potential (mainly on large scales) and by its effects on the density and velocity of ordinary matter. Ordinary and dark matter perturbations, therefore, evolve differently with time and leave different imprints on the CMB.

The CMB is very close to a perfect blackbody but contains very small temperature anisotropies of a few parts in 100,000. A sky map of anisotropies can be decomposed into an angular power spectrum, which is observed to contain a series of acoustic peaks at near-equal spacing but different heights. The locations of these peaks depend on cosmological parameters. Matching theory to data, therefore, constrains cosmological parameters.[81]

The CMB anisotropy was first discovered by COBE in 1992, though this had too coarse resolution to detect the acoustic peaks. After the discovery of the first acoustic peak by the balloon-borne BOOMERanG experiment in 2000, the power spectrum was precisely observed by WMAP in 2003–2012, and even more precisely by the Planck spacecraft in 2013–2015. The results support the Lambda-CDM model.[82][83]

The observed CMB angular power spectrum provides powerful evidence in support of dark matter, as its precise structure is well fitted by the Lambda-CDM model,[83] but difficult to reproduce with any competing model such as modified Newtonian dynamics (MOND).[84]

Structure formation

المقالة الرئيسية: تشكل البنية
3D map of the large-scale distribution of dark matter, reconstructed from measurements of weak gravitational lensing with the Hubble Space Telescope. [85]
ملف:Dark matter map of KiDS survey region (region G12).jpg
Dark matter map for a patch of sky based on gravitational lensing analysis of a Kilo-Degree Survey[86]

Structure formation refers to the period after the Big Bang when density perturbations collapsed to form stars, galaxies, and clusters. Prior to structure formation, the Friedmann solutions to general relativity describe a homogeneous universe. Later, small anisotropies gradually grew and condensed the homogeneous universe into stars, galaxies and larger structures. Ordinary matter is affected by radiation, which is the dominant element of the universe at very early times. As a result, its density perturbations are washed out and unable to condense into structure.[87] If there were only ordinary matter in the universe, there would not have been enough time for density perturbations to grow into the galaxies and clusters currently seen.

Dark matter provides a solution to this problem because it is unaffected by radiation. Therefore, its density perturbations can grow first. The resulting gravitational potential acts as an attractive potential well for ordinary matter collapsing later, speeding up the structure formation process.[87][88]


العملاق اللطيف

أول صورة لثقب "العملاق اللطيف" في درب التبانة.

في 12 مايو 2022، قدم العلماء أول نظرة على ما أسموه "العملاق اللطيف" المتربص في مركز درب التبانة، وكشفوا عن صورة لثقب أسود فائق الكتلة يلتهم أي مادة تتجول داخل جاذبيتها العملاقة.

الثقب الأسود - المسمى الرامي A* * - هو ثاني ثقب يتم تصويره على الإطلاق. تم إنجاز هذا العمل الفذ من خلال نفس التعاون الدولي ل"تلسكوب إڤنت هورايزون" الذي كشف في عام 2019 عن الصورة الأولى لثقب أسود - تلك الصورة الموجودة في قلب مجرة مختلفة.

في مؤتمر صحفي في واشنطن، أشادت فريال أوزيل، عالمة الفلك بجامعة أريزونا، ب"أول صورة مباشرة للعملاق اللطيف في وسط مجرتنا"، والتي تظهر حلقة متوهجة من الأحمر والأصفر والأبيض تحيط مركز أغمق.[89]


Sky surveys and baryon acoustic oscillations

المقالة الرئيسية: Baryon acoustic oscillations

Baryon acoustic oscillations (BAO) are fluctuations in the density of the visible baryonic matter (normal matter) of the universe on large scales. These are predicted to arise in the Lambda-CDM model due to acoustic oscillations in the photon–baryon fluid of the early universe and can be observed in the cosmic microwave background angular power spectrum. BAOs set up a preferred length scale for baryons. As the dark matter and baryons clumped together after recombination, the effect is much weaker in the galaxy distribution in the nearby universe, but is detectable as a subtle (~ 1%) preference for pairs of galaxies to be separated by 147 Mpc, compared to those separated by 130–160 Mpc. This feature was predicted theoretically in the 1990s and then discovered in 2005, in two large galaxy redshift surveys, the Sloan Digital Sky Survey and the 2dF Galaxy Redshift Survey.[90] Combining the CMB observations with BAO measurements from galaxy redshift surveys provides a precise estimate of the Hubble constant and the average matter density in the Universe.[91] The results support the Lambda-CDM model.

Theoretical classifications

المقالة الرئيسية: Cold dark matter, Warm dark matter, and Hot dark matter

Dark matter can be divided into cold, warm, and hot categories.[92] These categories refer to velocity rather than an actual temperature, and indicate how far corresponding objects moved due to random motions in the early universe, before they slowed due to cosmic expansion. This distance is called the free streaming length. The categories of dark matter are set with respect to the size of the collection of mass prior to structure formation that later collapses to form a dwarf galaxy. This collection of mass is sometimes called a protogalaxy. Dark matter particles are classified as cold, warm, or hot if their free streaming length is much smaller (cold), similar to (warm), or much larger (hot) than the protogalaxy of a dwarf galaxy.[93][94][95] Mixtures of the above are also possible: a theory of mixed dark matter was popular in the mid-1990s, but was rejected following the discovery of dark energy.[بحاجة لمصدر]

The significance of the free streaming length is that the universe began with some primordial density fluctuations from the Big Bang (in turn arising from quantum fluctuations at the microscale). Particles from overdense regions will naturally spread to underdense regions, but because the universe is expanding quickly, there is a time limit for them to do so. Faster particles (hot dark matter) can beat the time limit while slower particles cannot. The particles travel a free streaming length's worth of distance within the time limit; therefore this length sets a minimum scale for later structure formation. Because galaxy-size density fluctuations get washed out by free-streaming, hot dark matter implies the first objects that can form are huge supercluster-size pancakes, which then fragment into galaxies, while the reverse is true for cold dark matter.

Deep-field observations show that galaxies formed first, followed by clusters and superclusters as galaxies clump together,[55] and therefore that most dark matter is cold. This is also the reason why neutrinos, which move at nearly the speed of light and therefore would fall under hot dark matter, cannot make up the bulk of dark matter.[87]


Composition

ملف:Dark matter candidates.pdf
Different dark matter candidates as a function of their mass in units of electronvolt (eV)

The identity of dark matter is unknown, but there are many hypotheses about what dark matter could consist of, as set out in the table below.

Some dark matter hypotheses
light bosons quantum chromodynamics axions
axion-like particles
fuzzy cold dark matter
neutrinos Standard Model[ث]
sterile neutrinos
other particles lightest supersymmetric particle
weakly interacting massive particle
self-interacting dark matter
atomic dark matter[97][98][99][100]
strangelet[101]
dynamical dark matter[102]
macroscopic primordial black holes[17][18][103][19][104][105][106][107][108][109]
massive compact halo objects (MACHOs)
macroscopic dark matter (Macros)
Fermi-LAT observations of dwarf galaxies provide new insights on dark matter.

Baryonic matter

يجب ألا يـُخلـَط بينها وبين Missing baryon problem.

Dark matter can refer to any substance which interacts predominantly via gravity with visible matter (e.g., stars and planets). Hence in principle it need not be composed of a new type of fundamental particle but could, at least in part, be made up of standard baryonic matter, such as protons or neutrons. Most of the ordinary matter familiar to astronomers, including planets, brown dwarfs, red dwarfs, visible stars, white dwarfs, neutron stars, and black holes, fall into this category.[22][110] A black hole would ingest both baryonic and non-baryonic matter that comes close enough to its event horizon; afterwards, the distinction between the two is lost.[111]

These massive objects that are hard to detect are collectively known as MACHOs. Some scientists initially hoped that baryonic MACHOs could account for and explain all the dark matter.[52]:286[112]

However, multiple lines of evidence suggest the majority of dark matter is not baryonic:

  • Sufficient diffuse, baryonic gas or dust would be visible when backlit by stars.
  • The theory of Big Bang nucleosynthesis predicts the observed abundance of the chemical elements. If there are more baryons, then there should also be more helium, lithium and heavier elements synthesized during the Big Bang.[113][114] Agreement with observed abundances requires that baryonic matter makes up between 4–5% of the universe's critical density. In contrast, large-scale structure and other observations indicate that the total matter density is about 30% of the critical density.[78]
  • Astronomical searches for gravitational microlensing in the Milky Way found at most only a small fraction of the dark matter may be in dark, compact, conventional objects (MACHOs, etc.); the excluded range of object masses is from half the Earth's mass up to 30 solar masses, which covers nearly all the plausible candidates.[115][116][117][118][119][120]
  • Detailed analysis of the small irregularities (anisotropies) in the cosmic microwave background by WMAP and Planck indicate that around five-sixths of the total matter is in a form that only interacts significantly with ordinary matter or photons through gravitational effects.[121]

Non-baryonic matter

There are two main candidates for non-baryonic dark matter: new particles and primordial black holes.

Unlike baryonic matter, nonbaryonic particles do not contribute to the formation of the elements in the early universe (Big Bang nucleosynthesis)[122][123][53] and so its presence is felt only via its gravitational effects (such as weak lensing). In addition, some dark matter candidates can interact with themselves (self-interacting dark matter) or with ordinary particles (e.g. WIMPs or Weakly Interacting Massive Particles), possibly resulting in observable by-products such as gamma rays and neutrinos (indirect detection).[96] Candidates abound (see the table above), each with their own strengths and weaknesses.

Undiscovered massive particles

المقالة الرئيسية: Weakly Interacting Massive Particles

There exists no formal definition of a Weakly Interacting Massive Particle, but broadly, it is an elementary particle which interacts via gravity and any other force (or forces) which is as weak as or weaker than the weak nuclear force, but also non-vanishing in strength. Many WIMP candidates are expected to have been produced thermally in the early Universe, similarly to the particles of the Standard Model[124] according to Big Bang cosmology, and usually will constitute cold dark matter. Obtaining the correct abundance of dark matter today via thermal production requires a self-annihilation cross section of ≃ 3×10−26 cm3⋅s−1, which is roughly what is expected for a new particle in the 100 GeV/c2 mass range that interacts via the electroweak force.

Because supersymmetric extensions of the Standard Model of particle physics readily predict a new particle with these properties, this apparent coincidence is known as the "WIMP miracle", and a stable supersymmetric partner has long been a prime explanation for dark matter.[125] Experimental efforts to detect WIMPs include the search for products of WIMP annihilation, including gamma rays, neutrinos and cosmic rays in nearby galaxies and galaxy clusters; direct detection experiments designed to measure the collision of WIMPs with nuclei in the laboratory, as well as attempts to directly produce WIMPs in colliders, such as the Large Hadron Collider at CERN.

In the early 2010s, results from direct-detection experiments along with the lack of evidence for supersymmetry at the Large Hadron Collider (LHC) experiment[126][127] have cast doubt on the simplest WIMP hypothesis.[128]

Undiscovered ultralight particles

المقالة الرئيسية: Axion

Axions are hypothetical elementary particles originally theorized in 1978 independently by Frank Wilczek and Steven Weinberg as the Goldstone boson of Peccei–Quinn theory, which had been proposed in 1977 to solve the strong CP problem in quantum chromodynamics (QCD). QCD effects produce an effective periodic potential in which the axion field moves.[129] Expanding the potential about one of its minima, one finds that the product of the axion mass with the axion decay constant is determined by the topological susceptibility of the QCD vacuum. An axion with mass that is much less than 60 keV/c2 is long-lived and weakly interacting: a perfect dark matter candidate.

The oscillations of the axion field about the minimum of the effective potential, the so-called misalignment mechanism, generate a cosmological population of cold axions with an abundance depending on the mass of the axion.[130][131][132] With a mass above 5 μeV/c2 (10−11 times the electron mass) axions could account for dark matter, and thus be both a dark-matter candidate and a solution to the strong CP problem. If inflation occurs at a low scale and lasts sufficiently long, the axion mass can be as low as 1 peV/c2.[133][134][135]

Because axions have extremely low mass, their de Broglie wavelength is very large, in turn meaning that quantum effects could help resolve the small-scale problems of the Lambda-CDM model. A single ultralight axion with a decay constant at the grand unified theory scale provides the correct relic density without fine-tuning.[136]

Axions as a dark matter candidate have gained in popularity in recent years, because of the non-detection of WIMPS.[137]

Primordial black holes

المقالة الرئيسية: Primordial black hole

Primordial black holes are hypothetical black holes that formed soon after the Big Bang. In the inflationary era and early radiation-dominated universe, extremely dense pockets of subatomic matter may have been tightly packed to the point of gravitational collapse, creating primordial black holes without the supernova compression typically needed to make black holes today. The idea that black holes could form in the early universe was first suggested by Yakov Zeldovich and Igor Dmitriyevich Novikov in 1967, and independently by Stephen Hawking in 1971. It quickly became clear that such black holes might account for at least part of dark matter. Because the creation of primordial black holes would pre-date the first stars, they are not limited to the narrow mass range of stellar black holes and also not classified as baryonic dark matter.

Primordial black holes as a dark matter candidate has the major advantage that it is based on a well-understood theory (General Relativity) and objects (black holes) that are already known to exist. However, producing primordial black holes requires exotic cosmic inflation or physics beyond the standard model of particle physics,[138] and might also require fine-tuning.[139] Primordial black holes can also span nearly the entire possible mass range, from atom-sized to supermassive.

The idea that primordial black holes make up dark matter gained prominence in 2015[140] following results of gravitational wave measurements which detected the merger of intermediate-mass black holes. Black holes with about 30 solar masses are not predicted to form by either stellar collapse (typically less than 15 solar masses) or by the merger of black holes in galactic centers (millions or billions of solar masses), which suggests that the detected black holes might be primordial. A later survey of about a thousand supernovae detected no gravitational lensing events, when about eight would be expected if intermediate-mass primordial black holes above a certain mass range accounted for over 60% of dark matter.[141] However, that study assumed that all black holes have the same or similar mass to the LIGO/Virgo mass range, which might not be the case (as suggested by subsequent James Webb Space Telescope observations).[142][103]

The possibility that atom-sized primordial black holes account for a significant fraction of dark matter was ruled out by measurements of positron and electron fluxes outside the Sun's heliosphere by the Voyager 1 spacecraft. Tiny black holes are theorized to emit Hawking radiation. However the detected fluxes were too low and did not have the expected energy spectrum, suggesting that tiny primordial black holes are not widespread enough to account for dark matter.[143] Nonetheless, research and theories proposing dense dark matter accounts for dark matter continue as of 2018, including approaches to dark matter cooling,[144][145] and the question remains unsettled. In 2019, the lack of microlensing effects in the observation of Andromeda suggests that tiny black holes do not exist.[146]

Nonetheless, there still exists a largely unconstrained mass range smaller than that which can be limited by optical microlensing observations, where primordial black holes may account for all dark matter.[147][148]

Modified gravity

للمزيد من المعلومات: Alternatives to general relativity

The last major possibility is that general relativity, the theory underpinning modern cosmology, is incorrect. General relativity is well-tested on Solar System scales, but its validity on galactic or cosmological scales has not been well proven.[149] A suitable modification to general relativity can conceivably eliminate the need for dark matter. The best-known theories of this class are MOND and its relativistic generalization tensor–vector–scalar gravity (TeVeS),[150] f(R) gravity,[151] negative mass, dark fluid,[152][153][154] and entropic gravity.[155] Alternative theories abound.[156][157]

A problem with modifying gravity is that observational evidence for dark matter – let alone general relativity – comes from so many independent approaches (see the "observational evidence" section above). Explaining any individual observation is possible but explaining all of them in the absence of dark matter is very difficult. Nonetheless, there have been some scattered successes for alternative hypotheses, such as a 2016 test of gravitational lensing in entropic gravity[158][159][160] and a 2020 measurement of a unique MOND effect.[161][162]

The prevailing opinion among most astrophysicists is that while modifications to general relativity can conceivably explain part of the observational evidence, there is probably enough data to conclude there must be some form of dark matter present in the universe.[21]

Dark matter aggregation and dense dark matter objects

If dark matter is composed of weakly interacting particles, then an obvious question is whether it can form objects equivalent to planets, stars, or black holes. Historically, the answer has been it cannot,[ج][163][164][165] because of two factors:

It lacks an efficient means to lose energy[163]
Ordinary matter forms dense objects because it has numerous ways to lose energy. Losing energy would be essential for object formation, because a particle that gains energy during compaction or falling "inward" under gravity, and cannot lose it any other way, will heat up and increase velocity and momentum. Dark matter appears to lack a means to lose energy, simply because it is not capable of interacting strongly in other ways except through gravity. The virial theorem suggests that such a particle would not stay bound to the gradually forming object – as the object began to form and compact, the dark matter particles within it would speed up and tend to escape.
It lacks a diversity of interactions needed to form structures[165]
Ordinary matter interacts in many different ways, which allows the matter to form more complex structures. For example, stars form through gravity, but the particles within them interact and can emit energy in the form of neutrinos and electromagnetic radiation through fusion when they become energetic enough. Protons and neutrons can bind via the strong interaction and then form atoms with electrons largely through electromagnetic interaction. There is no evidence that dark matter is capable of such a wide variety of interactions, since it seems to only interact through gravity (and possibly through some means no stronger than the weak interaction, although until dark matter is better understood, this is only speculation).

Detection of dark matter particles

If dark matter is made up of subatomic particles, then millions, possibly billions, of such particles must pass through every square centimeter of the Earth each second.[166][167] Many experiments aim to test this hypothesis. Although WIMPs have been the main search candidates,[55] axions have drawn renewed attention, with the Axion Dark Matter Experiment (ADMX) searches for axions and many more planned in the future.[168] Another candidate is heavy hidden sector particles which only interact with ordinary matter via gravity.

These experiments can be divided into two classes: direct detection experiments, which search for the scattering of dark matter particles off atomic nuclei within a detector; and indirect detection, which look for the products of dark matter particle annihilations or decays.[96]

Direct detection

للمزيد من المعلومات: Weakly interacting massive particle § Direct detection
المقالة الرئيسية: Direct detection of dark matter

Direct detection experiments aim to observe low-energy recoils of nuclei (typically a few keV) induced by interactions with particles of dark matter, which (in theory) are passing through the Earth. After such a recoil, the nucleus will emit energy in the form of scintillation light or phonons as they pass through sensitive detection apparatus. To do so effectively, it is crucial to maintain an extremely low background, which is the reason why such experiments typically operate deep underground, where interference from cosmic rays is minimized. Examples of underground laboratories with direct detection experiments include the Stawell mine, the Soudan mine, the SNOLAB underground laboratory at Sudbury, the Gran Sasso National Laboratory, the Canfranc Underground Laboratory, the Boulby Underground Laboratory, the Deep Underground Science and Engineering Laboratory and the China Jinping Underground Laboratory.

These experiments mostly use either cryogenic or noble liquid detector technologies. Cryogenic detectors operating at temperatures below 100 mK, detect the heat produced when a particle hits an atom in a crystal absorber such as germanium. Noble liquid detectors detect scintillation produced by a particle collision in liquid xenon or argon. Cryogenic detector experiments include such projects as CDMS, CRESST, EDELWEISS, and EURECA, while noble liquid experiments include LZ, XENON, DEAP, ArDM, WARP, DarkSide, PandaX, and LUX, the Large Underground Xenon experiment. Both of these techniques focus strongly on their ability to distinguish background particles (which predominantly scatter off electrons) from dark matter particles (that scatter off nuclei). Other experiments include SIMPLE and PICASSO, which use alternative methods in their attempts to detect dark matter.

Currently there has been no well-established claim of dark matter detection from a direct detection experiment, leading instead to strong upper limits on the mass and interaction cross section with nucleons of such dark matter particles.[169] The DAMA/NaI and more recent DAMA/LIBRA experimental collaborations have detected an annual modulation in the rate of events in their detectors,[170][171] which they claim is due to dark matter. This results from the expectation that as the Earth orbits the Sun, the velocity of the detector relative to the dark matter halo will vary by a small amount. This claim is so far unconfirmed and in contradiction with negative results from other experiments such as LUX, SuperCDMS[172] and XENON100.[173]

A special case of direct detection experiments covers those with directional sensitivity. This is a search strategy based on the motion of the Solar System around the Galactic Center.[174][175][176][177] A low-pressure time projection chamber makes it possible to access information on recoiling tracks and constrain WIMP-nucleus kinematics. WIMPs coming from the direction in which the Sun travels (approximately towards Cygnus) may then be separated from background, which should be isotropic. Directional dark matter experiments include DMTPC, DRIFT, Newage and MIMAC.

Indirect detection

المقالة الرئيسية: Indirect detection of dark matter
ملف:Collage of six cluster collisions with dark matter maps.jpg
Collage of six cluster collisions with dark matter maps. The clusters were observed in a study of how dark matter in clusters of galaxies behaves when the clusters collide.[178]
ملف:Turning Black Holes into Dark Matter Labs.webm
Video about the potential gamma-ray detection of dark matter annihilation around supermassive black holes. (Duration 0:03:13, also see file description.)

Indirect detection experiments search for the products of the self-annihilation or decay of dark matter particles in outer space. For example, in regions of high dark matter density (e.g., the centre of the Milky Way) two dark matter particles could annihilate to produce gamma rays or Standard Model particle–antiparticle pairs.[179] Alternatively, if a dark matter particle is unstable, it could decay into Standard Model (or other) particles. These processes could be detected indirectly through an excess of gamma rays, antiprotons or positrons emanating from high density regions in the Milky Way and other galaxies.[180] A major difficulty inherent in such searches is that various astrophysical sources can mimic the signal expected from dark matter, and so multiple signals are likely required for a conclusive discovery.[55][96]

A few of the dark matter particles passing through the Sun or Earth may scatter off atoms and lose energy. Thus dark matter may accumulate at the center of these bodies, increasing the chance of collision/annihilation. This could produce a distinctive signal in the form of high-energy neutrinos.[181] Such a signal would be strong indirect proof of WIMP dark matter.[55] High-energy neutrino telescopes such as AMANDA, IceCube and ANTARES are searching for this signal.[52]:298 The detection by LIGO in September 2015 of gravitational waves opens the possibility of observing dark matter in a new way, particularly if it is in the form of primordial black holes.[182][183][184]

Many experimental searches have been undertaken to look for such emission from dark matter annihilation or decay, examples of which follow.

The Energetic Gamma Ray Experiment Telescope observed more gamma rays in 2008 than expected from the Milky Way, but scientists concluded this was most likely due to incorrect estimation of the telescope's sensitivity.[185]

The Fermi Gamma-ray Space Telescope is searching for similar gamma rays.[186] In 2009, an as yet unexplained surplus of gamma rays from the Milky Way's galactic center was found in Fermi data. This Galactic Center GeV excess might be due to dark matter annihilation or to a population of pulsars.[187] In April 2012, an analysis of previously available data from Fermi's Large Area Telescope instrument produced statistical evidence of a 130 GeV signal in the gamma radiation coming from the center of the Milky Way.[188] WIMP annihilation was seen as the most probable explanation.[189]

At higher energies, ground-based gamma-ray telescopes have set limits on the annihilation of dark matter in dwarf spheroidal galaxies[190] and in clusters of galaxies.[191]

The PAMELA experiment (launched in 2006) detected excess positrons. They could be from dark matter annihilation or from pulsars. No excess antiprotons were observed.[192]

In 2013, results from the Alpha Magnetic Spectrometer on the International Space Station indicated excess high-energy cosmic rays which could be due to dark matter annihilation.[193][194][195][196][197][198]

Collider searches for dark matter

An alternative approach to the detection of dark matter particles in nature is to produce them in a laboratory. Experiments with the Large Hadron Collider (LHC) may be able to detect dark matter particles produced in collisions of the LHC proton beams. Because a dark matter particle should have negligible interactions with normal visible matter, it may be detected indirectly as (large amounts of) missing energy and momentum that escape the detectors, provided other (non-negligible) collision products are detected.[199] Constraints on dark matter also exist from the LEP experiment using a similar principle, but probing the interaction of dark matter particles with electrons rather than quarks.[200] Any discovery from collider searches must be corroborated by discoveries in the indirect or direct detection sectors to prove that the particle discovered is, in fact, dark matter.

In popular culture

Dark matter regularly appears as a topic in hybrid periodicals that cover both factual scientific topics and science fiction,[201] and dark matter itself has been referred to as "the stuff of science fiction".[202]

Mention of dark matter is made in works of fiction. In such cases, it is usually attributed extraordinary physical or magical properties, thus becoming inconsistent with the hypothesized properties of dark matter in physics and cosmology. For example:

More broadly, the phrase "dark matter" is used metaphorically in fiction to evoke the unseen or invisible.[206]

Gallery

See also

Related theories
Experiments
Dark matter candidates
Other


Notes

  1. ^ "Um, wie beobachtet, einen mittleren Dopplereffekt von 1000 km/sek oder mehr zu erhalten, müsste also die mittlere Dichte im Comasystem mindestens 400 mal grösser sein als die auf Grund von Beobachtungen an leuchtender Materie abgeleitete. Falls sich dies bewahrheiten sollte, würde sich also das überraschende Resultat ergeben, dass dunkle Materie in sehr viel grösserer Dichte vorhanden ist als leuchtende Materie."[33]:{{{1}}}
    [In order to obtain an average Doppler effect of 1000 km/s or more, as observed, the average density in the Coma system would thus have to be at least 400 times greater than that derived on the basis of observations of luminous matter. If this were to be confirmed, the surprising result would then follow that dark matter is present in very much greater density than luminous matter.]
  2. ^ However, in the modern cosmic era, this neutrino field has cooled and started to behave more like matter and less like radiation.
  3. ^ This is a consequence of the shell theorem and the observation that spiral galaxies are spherically symmetric to a large extent (in 2D).
  4. ^ The three neutrino types already observed are indeed abundant, and dark, and matter, but their individual masses are almost certainly too tiny to account for more than a small fraction of dark matter, due to limits derived from large-scale structure and high-redshift galaxies.[96]
  5. ^ "One widely held belief about dark matter is it cannot cool off by radiating energy. If it could, then it might bunch together and create compact objects in the same way baryonic matter forms planets, stars, and galaxies. Observations so far suggest dark matter doesn't do that – it resides only in diffuse halos ... As a result, it is extremely unlikely there are very dense objects like stars made out of entirely (or even mostly) dark matter." — Buckley & Difranzo (2018)[163]

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Further reading

External links

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