Phys/Astro 689: Lecture 1. Evidence for Dark Matter

Transcription

1 Phys/Astro 689: Lecture 1 Evidence for Dark Matter

2 Why? This class is primarily a consideration of whether Cold Dark Matter theory can be reconciled with galaxy observations. Spoiler: CDM has a small scale crisis. Why do we take CDM so seriously?

3 Why? cont d A lot of people don t like the idea of Dark Matter. As (astro)physicists, we should understand the evidence and be able to state why the existence of DM is so compelling.

4 The Evidence Galaxy Dynamics CMB Power Spectrum BBN (Big Bang Nucleosynthesis) Bullet Cluster

5 1930 s: Velocity Dispersions of Clusters WHERE NEBULAE = GALAXIES

6 Velocity Dispersions of Clusters From Virial Theorem Formally, Virial relations: for a dispersion supported system

7 1970 s: Rotation Curves of Spiral Galaxies SEE ALSO: VAN DE HULST (1957) KAHN & WOLTJER (1959) BY 1980 S MOST ASTRONOMERS BELIEVE IN DM IN GALAXIES

8 Rotation Curves of Spiral Galaxies v c 2 = GM(<r)/r for a rotation supported system HI GAS + STARS

9 Additional Stability Argument UNSTABLE TO DEVELOPMENT OF BARS SEE ALSO: EFSTATHIOU, LAKE, & NEGROPONTE (1982)

10 The Evidence Galaxy Dynamics CMB Power Spectrum BBN (Big Bang Nucleosynthesis) Bullet Cluster

11 Understanding the CMB Power Spectrum image courtesy of WMAP webpage

12 Timeline of the Early Universe

13 Prior to seconds Quantum fluctuations Horizon set by speed of light and age of Universe (no information transferred past horizon) Fluctuations lead to acoustic oscillations Inflation causes acoustic oscillations to be in phase HU & WHITE, FEBRUARY 2004, SCIENTIFIC AMERICAN

14 Acoustic Oscillations Pick a region with the maximum displacement due to the acoustic wave (hottest region) Begins to oscillate toward minimum displacement Wave that causes region to reach minimum at recombination is the fundamental mode Overtones are integer fractions of fundamental mode HU & WHITE, FEBRUARY 2004, SCIENTIFIC AMERICAN

15 Acoustic Oscillations Peaks in power spectrum are evidence for in phase sound waves (else they would blur), and hence inflation

16 First Peak Only ordinary matter undergoes the acoustic oscillations, but Dark Matter adds gravity (and hence extra compression) First peak: region that was maximally hot at inflation is now maximally dense. Gravity enhances compressed region HU & WHITE, FEBRUARY 2004, SCIENTIFIC AMERICAN

17 Second Peak First overtone/2nd peak: While there s some gravitational compression, gas pressure is pushing back. So amplitude (temperature change) is lower than first peak. Ratio of 1st to 2nd peak then supplies information on baryon to DM ratio HU & WHITE, FEBRUARY 2004, SCIENTIFIC AMERICAN

18 Understanding the CMB Power Spectrum Summary: the CMB power spectrum tells us how much matter (gravitational compression) there was in the early Universe, but also how much pressure from baryons. Baryons can t contribute all of the gravity, so some other form of matter must exist image courtesy of WMAP webpage

19 The Evidence Galaxy Dynamics CMB Power Spectrum BBN (Big Bang Nucleosynthesis) Bullet Cluster

20 Do we need BBN? Could stars produce the He we observe? 4p --> 4 He+2e + +2ν e Δm = 25.7 MeV Using flux from Sun at Earth, mass of Sun, and adopting an age of 5Gyr, find: ~5% of the Sun s mass could be converted to He so far Observationally, the Universe is ~25% He by mass, ~75% H

21 Do we need BBN? Neutrons are another important clue free neutrons β decay with τ1/2 ~ 10min What is n/p ratio today?

22 Do we need BBN? Neutrons are another important clue free neutrons β decay with τ1/2 ~ 10min What is n/p ratio today? He has 2n + 2p = 4 mass units. H has 3x as many mass units = 12p. 2n/14p = 0.14

23 How Does the Process Begin? Densities are too low for more than 2-body interactions Deuteron is only bound 2-nucleon state n+p <--> d+, so assume formation time is when densities are roughly equal: Separately: Recall E density of black body: Temp of CMB tells us E density of photons; plus info on baryons, so can derive

24 Predictions and Tests

25 BBN Summary The predicted abundances are sensitive to the baryon-to-photon ratio, so tell us how much matter is in baryons. In other words, we constrain baryons independently, but still need more matter (that doesn t participate in nucleosynthesis) to match other observations

26 The Evidence Galaxy Dynamics CMB Power Spectrum BBN (Big Bang Nucleosynthesis) Bullet Cluster

27 Bullet Cluster X-RAY: RED DM DISTRIBUTION: BLUE (FROM LENSING)

28 What do we know about Dark Matter? It interacts gravitationally: therefore it has mass It s neutral: it doesn t participate in electromagnetic interactions. Doesn t participate in nucleosynthesis, so it s not dark baryons.

29 The Big DM Unknown What s its mass?!?!?!!!??