Dark radiation in cosmology 张益 重庆邮电大学 2011.4.3

Dark Energy! Dark Matter! What is dark radiation？ Observations! Theory!

宇宙中的组分

不同组分的演化规律 Friedmann Equation Radiation Matter Cosmological constant Curvature term

集中讨论两个加速膨胀阶段

Radiation 通常: 辐射是指静止质量为零的粒子组成的气体 在温度为T的早期宇宙中： 当温度T显著超过粒子静质量m，这种粒子会作为辐射的组分存在； 辐射气体是一切满足m<T的粒子组成的混合气体。

辐射物质 光子， 正反中微子， 正反电子， 质子和中子

BBN（Big Bang Nucleosynthesis） 热碰撞(1MeV) 温度降到0.1MeV： 电子俘获 碰撞 Next 温度降到0.01MeV, 热核反应结束

The radiation dominating universe

辐射项对BBN 的影响 有效中微子数目大 宇宙的膨胀率大 弱退耦发生的时间得更早 冻结下来的中子数密度更多 氦的产额更大

CMBR（微波背景辐射） CMBR各向异性可分为： 宇宙早期产生的初级各向异性 从光子退耦到现在产生的二级各向异性

ISW 效应（ Integrated Sachs-Wolfe effect ） 宇宙不是完全均匀的，所以到处存在着引力势阱。当微波背景辐射的光子掉进引力势阱的时候就会获得能量，爬出这个势阱的时候就会损失能量。 该效应是光子路径上所有引力势变化的累加效应，称为积分Sachs-Wolfe效应.

1, 对于一个固定的能量密度 辐射能量密度增加 物质-辐射相等的时刻延迟 ISW效应加强 2, 没有和光子-重子流体耦合的相对论性粒子，可以比声速更快地逃出势阱。

Motivation from Observations WMAP shows: BBN Observations limit（68% CL) (astro-ph:0408033） CMBR （68% CL) （ arxiv:1001.4538， 1009.0866 ） ATACAMA COSMOLOGY TELESCOPE: ACT measures fluctuations at scale 500<l<3000

Possibilities!（ astro-ph:0612150,1103.4132） Motivated by the LSND(Liquid Scintillator Neutrino Detector) oscillation claim，assume a (light) massive sterile neutrino (1006.5276); The neutrinos acquire a mass from a broken lepton flavor symmetry(astroph:0312267,1009.4666,1011.0911); Allow for a violation of the spin-statistics theorem; A Brans-Dicke field with non trivial potential which could mimic the effect of adding extra radiation between the BBN and CMB epochs（astro-ph:0510359）; An extra interaction between the dark energy and radiation (or dark matter); A quintessence with a tracking potential behavior; Decaying matter(1011.3501); Nonstandard thermal history; ……

Theories Brane Cosmology Hovara-Lifshitz cosmology Electroweak phase transition EDE（Early Dark Energy） ………..

Brane Cosmology（astro-ph:0211285） Dark radiation term arises from the projection of Weyl curvature of the bulk black hole on the brane and behaves like an additional collisionless and isotropic massless component.

其中BBN的限制是 CMB的限制：把CMB的峰移到高l处 对于

Electroweak phase transition (0902.4699)

Triple coincidence problem Coincidence problem: why the cosmological constant and the matter have comparable energy density today even through their time evolution is so different Triple coincidence problem: why the radiation energy density today is only three orders of magnitude smaller than the dark-matter and dark-energy ones, although it also scales very differently

Scalar field dark energy Interaction 1 Interaction 2

Phase-space analysis

Quintessence

Phantom

点

Conclusion “Triple coincidence problem”still need fine-tunning。 This model can recover the history of our universe.

The Future Data：Planck, BAO, new result on primordial nuclei abundance? Models: based on string theory, particle physics or others?

THANKS ALL !