Dark Matter and Dark Energy components chapter 7

Transcription

1 Dark Matter and Dark Energy components chapter 7 Lecture 4 See also Dark Matter awareness week December

2 The early universe chapters 5 to 8 Particle Astrophysics, D. Perkins, 2 nd edition, Oxford 5. The expanding universe 6. Nucleosynthesis and baryogenesis 7. Dark matter and dark energy components 8. Development of structure in early universe Slides + book

3 Overview Part 1: Observation of dark matter as gravitational effects Rotation curves galaxies, mass/light ratios in galaxies Velocities of galaxies in clusters Gravitational lensing Bullet cluster Part 2: Nature of the dark matter : Baryons and MACHO s Standard neutrinos Axions Part 3: Weakly Interacting Massive Particles (WIMPs) Part 4: Experimental WIMP searches Part 5: Dark energy Dark Matter - Dark Energy 3

4 Universe is flat k=0 Previously ΛCDM model Dynamics given by Friedman equation H t R t G N R t 3 ρtot Size of universe at given time and today, t 0 Closure parameter relates t t density to critical density c t Energy density evolves with time, and so does H t Rt 0 1 z z t Ω t Ω t0 0 1 H t H z t z t z m r tot mat rad Λ Rt k Ω k = Dark Matter - Dark Energy 4

5 Energy budget of universe today Today only 5% of the matterenergy consists of known particles luminous 1% dark baryonic 4% Neutrino HDM <1% cold dark matter 18% 18% is Cold Dark Matter = new type of particles dark energy 76% 76% is completely unknown H t H t z t z t m r rad matter Dark Matter - Dark Energy 5

6 Previous lecture : Dark Matter Observation of dark matter as gravitational effects Nature of dark matter particles is still inknown Baryons MACHOs = Massive Compact Halo Objects Neutrinos Axions WIMPs = Weakly Interacting Massive Particles Identifying dark matter : direct and indirect detection experiments Ω m Dark Matter - Dark Energy 6

7 Where should we look? Search for WIMPs in the Milky Way halo Indirect detection: expect WIMPs from the halo to annihilate with each other to known particles Direct detection: expect WIMPs from the halo to interact in a detector on Earth Solar system Dark matter halo Luminous disk ESO Dark Matter - Dark Energy 7

8 Identify the nature of dark matter with experiments PART 4: WIMP DETECTION Dark Matter - Dark Energy 8

9 DIRECT DETECTION EXPERIMENTS Dark Matter - Dark Energy 9

10 Principle of direct detection Earth moves in WIMP wind from halo Elastic collision of WIMP with nucleus in detector N N recoil energy 2 2 v µ ERec 1 cos 50keV m N Velocity of WIMPs ~ velocity of galactic objects v χ 1 3 ~ 270 km s ~ 10 c µ m m m N m N Dark Matter - Dark Energy 10

11 DM Density (GeV cm -3 ) Cross section and event rates Event rate depends on density of WIMPs in solar system R N m σ χp 3 ~ 0.3GeV cm Rate depends on number N of nuclei in target Distance from centre (kpc) Rate depends on scattering cross section present upper limit σ χp 10 cm 10 pb Weak interactions! Dark Matter - Dark Energy 11

12 Direct detection challenges R N M p low rate large detector very small signal low threshold large background : protect against cosmic rays, radioactivity, Dark Matter - Dark Energy 12

13 Annual modulation Annual modulations due to movement of solar system in galactic WIMP halo 30% effect Observed by DAMA/LIBRA not confirmed by other experiments Against the wind in June higher rate R N M p v ~ 270 km s 1 In direction of the wind in December Lower rate Dark Matter - Dark Energy 13

14 From event rate to cross section R N σ χp M Some experiments claim to see a signal at this mass and with this cross section Other experiments see no signal and put upper limits on the cross section Expected cross sections for models with supersymmetry Dark Matter - Dark Energy 14

15 INDIRECT DETECTION EXPERIMENTS Dark Matter - Dark Energy 15

16 Indirect detection of WIMPs Search for signals of annihilation of WIMPs in the Milky Way halo accumulation near galactic centre or in heavy objects like the Sun or Earth due to gravitational attraction Detect the produced antiparticles, gamma rays, neutrinos W qq ll, Z, H,, e, Dark Matter - Dark Energy 16

17 neutrinos from WIMP annihilations : IceCube Neutrinos from WIMP annihilations in the Sun Neutrinos from WIMP annihilations in galactic halo, galactic centre, dwarf spheroidal galaxies Neutrinos from WIMP annihilations in the centre of the Earth Dark Matter - Dark Energy 17

18 WIMPs accumulated in the Sun 1. WIMPs σ χn are captured C SUN 3 2 Capture rate v m μ ν interaction qq, ll, WIMPs annihilate ANN 1 2 CSUN Annihilation rate Detector Dark Matter - Dark Energy 18

19 Search for GeV-TeV neutrinos from Sun BG A few atmospheric neutrinos per year from northern hemisphere signal Max. a few neutrinos per year from WIMPs in the Sun BG ~10 11 atmospheric muons per year from southern hemisphere Dark Matter - Dark Energy 19

20 Signal? Number of events No excess above known backgrdound Data Background ψ(deg) No significant signal found Rate is compatible with atmospheric background Set upper limit on possible neutrino flux and annihilation rate Dark Matter - Dark Energy 20

21 Combining direct and IceCube searches 2 SD p cm IceCube W W Ann ~ C ~ σ SUN χn A selection of SuperSymmetry models Dark Matter - Dark Energy 21

22 Overview Part 1: Observation of dark matter as gravitational effects Rotation curves galaxies, mass/light ratios in galaxies Velocities of galaxies in clusters Gravitational lensing Bullet cluster Part 2: Nature of the dark matter : Baryons and MACHO s Standard neutrinos Axions Part 3: Weakly Interacting Massive Particles (WIMPs) Part 4: Experimental WIMP searches Part 5: Dark energy Dark Matter - Dark Energy 22

23 Part 5: Dark Energy Observations The nature of Dark Energy Ω Λ

24 Energy budget of universe today Today only 5% of the matterenergy consists of known particles luminous 1% dark baryonic 4% Neutrino HDM <1% cold dark matter 18% 18% is Cold Dark Matter = new type of particles dark energy 76% 76% is completely unknown H t H t z t z t m r rad matter Dark Matter - Dark Energy 24

25 SNIa surveys Link to cosmological parameters OBSERVATIONS Dark Matter - Dark Energy 25

26 Nobel Prize in Physics Dark Matter - Dark Energy 26

27 SNIa as standard candles (see lecture 1) Supernovae Ia are very bright - very distant SN can be observed All have roughly the same luminosity curve which allows to extract the absolute magnitude M effective magnitudes yield luminosity distance M 2.5log L cst m Network of telescopes united in z M 5log L Mpc Supernova Cosmology Project SCP, (Perlmutter) up to z=1.4 have now data from 500 SN ( High-z SN search HZSNS (Schmidt & Riess, in 1990 s) Dark Matter - Dark Energy 27 D

28 Luminosity distance vs redshift - 1 SN at redshift z emits light at time t E is observed at time t 0 Light observed today travelled during time (t 0 - t E ) distance travelled depends on history of expansion Proper distance D H from SN to Earth (lecture 1, horizon distance) t0 cdt dz D H te R t dt 0 te Rt 1 z H For flat universe (k=0) with negligible radiation content ( ) H z H t H t z t m Dark Matter - Dark Energy 28

29 Luminosity distance vs redshift - 2 D H z z z cdz c dz H z H 3 t 1 z t m Luminosity distance DL z 1 z DH z Dark Matter - Dark Energy 29

30 Luminosity distance D L vs redshift z Neglect radiation at low z different examples Dark Matter - Dark Energy 30

31 normalise to empty universe normalise measurements to empty universe k t 1, t 0, t 0 0 m 0 0 z c dz c z DL empty 1 z z 1 1 H H0 2 1 z Deceleration parameter is zero universe is coasting m t q t r t t 0 2 qt R R R R 2 R R R Dark Matter - Dark Energy 31 2 R = 0

32 Log D L (Mpc) Hubble plot with high z SNIa Vacuum dominated & flat Empty universe: Ω m = Ω r =0 Ω k =1 No acceleration no deceleration Matter dominated & flat now Redshift Z past Dark Matter - Dark Energy 32

33 Influence of cosmological parameters best fit best fit Empty universe measurements Empty universe Difference between measured logd L vs z and expected logd L vs z for empty universe Dark Matter - Dark Energy 33

34 Measurements up to z=1.7 Best fit m t t log D measured log D empty L L Δ(m-M)=ΔD L q(t)<0 acceleration --- Best fit q(t)>0 deceleration. empty present Dark Matter - Dark Energy 34 z past

35 SN observations show acceleration m t q t r t t 2 Early universe : universe dominated by matter decelerates due to gravitational collapse : q t t 0 R 0 Recently : universe dominated by vacuum energy accelerates Acceleration = zero around z=0.5 when m 2 t Switch from deceleration (matter dominated) to acceleration (dark energy dominated) Dark Matter - Dark Energy 35 m q t t 0 R 0 r t t

36 Related to cosmological constant? problems THE NATURE OF DARK ENERGY Dark Matter - Dark Energy 36

37 Dark vacuum Energy Observed present-day acceleration means that Dark Energy generates a negative pressure Yields gravitational repulsion like vacuum energy Equation of state (see lecture 1) E V 2 Pvac c cst S w P c 2 Fits to SNIa data yield that w is compatible with vacuum energy w 0.85 at 95% C. L Dark Matter - Dark Energy 37

38 Related to cosmological constant Λ In ΛCDM Ω Λ is constant and related to Einsteins cosmological constant If Universe contains constant vacuum energy density related to Λ then we set 8 G vac In very early Universe there was only vacuum energy At Planck energy scale gravity becomes strong energy and length scales 1 def c 19 M PLc GeV G And expected energy density L PL vac DEF c M PL c M c 10 exp 3 L 35 m 2 2 PL PL GeV m Dark Matter - Dark Energy 38

39 Cosmological constant problem Today vacuum energy density is of order of critical density vac c 0.76 c 5GeV m obs crit 10 GeV m vacc exp Discrepancy of 100 orders of magnitude!!! Did vacuum energy evolve with time? There is no explanation yet need high statistics data : Dark Energy Survey start 2011 ( WFIRST mission of NASA launch 2020 ( ) ESA EUCLID mission launch ( ) Dark Matter - Dark Energy 39

40 Alternatives Quintessence 5th force : acceleration is due to potential energy of a dynamical field Vacuum energy density varies with time Mechanism analogue to inflation in early universe P c 2 wt 1 wt 1 3 Maybe general relativity works differently at large distances Dark Matter - Dark Energy 40

41 Summary Observations of SNIa emission show that the universe presently accelerates This can be explained by a constant dark vacuum energy contribution which dominates today In the ΛCDM model the dark energy is related to the cosmological constant introduced by Einstein There is yet no satisfactory explanation for the observed magnitude of the dark energy Dark Matter - Dark Energy 41

42 Overview Part 1: Observation of dark matter as gravitational effects Rotation curves galaxies, mass/light ratios in galaxies Velocities of galaxies in clusters Gravitational lensing Bullet cluster Part 2: Nature of the dark matter : Baryons and MACHO s Standard neutrinos Axions Part 3: Weakly Interacting Massive Particles (WIMPs) Part 4: Experimental WIMP searches Part 5: Dark energy Dark Matter - Dark Energy 42