Physical Cosmology 4/4/2016. Docente: Alessandro Melchiorri

Transcription

1 Physical Cosmology 4/4/2016 Docente: Alessandro Melchiorri

2 Suggested textbooks Barbara Ryden, Introduction to Cosmology

3 Suggested textbooks An introduction to General Relativity, Sean Carroll

4 Suggested textbooks Modern Cosmology, Scott Dodelson

5 Suggested textbooks T. Padmanabhan, structure formation in the universe

6 Cosmological Constant - Current cosmological data suggest the presence of a cosmological constant at high significance. Assuming a flat universe, as confirmed by CMB observations (we will see this in a future lecture), SN-Ia (JLA) data gives: - But a cosmological constant is of extremely difficult theoretical interpretation! orders of magnitude difference (smaller) with the vacuum fluctuations energy expected in Quantum Field Theory! - Why now problem? why we live with a cosmological constant today?

7 Major goal of modern cosmology - Do we really need a cosmological constant? - Maybe data could be explained by a different component? - We need to falsify a cosmological constant! - A cosmological constant has: Constant with time (redshift) energy density We need to test these two things! Constant with time (redshift) equation of state and equal to -1!

8 Dark Energy As a first step we can fit the data with a component with a generic equation of state w constant with redshift. From the continuity equation. As we can see if w is different from -1 energy density is evolving with z! Assuming a flat universe, current SN-Ia(JLA)+CMB data gives: Very close to a cosmological constant!!!!

9 Dark Energy with CPL But, in principle, the equation of state must be redshift dependent if we want at least to address the why now? problem! A possible (widely used) parametrization is the CPL parametrization: (Integrating the continuity equation) This model has an equation of state equal to w0 at low redshift that converges to w0+wa at high redshifts. It recovers a cosmological constant for w0=-1 and wa=0.

10 Constraints on CPL Betoule et al., 2014 Gray region is SN-Ia (JLA) + CMB (Planck+WP) + Galaxy Clustering (BAO) Cosmological constant (two dashed lines) is ok. Constraints are weaker on w0 respect to w constant. Constraints on wa are very weak!!!

11 More General Parametrizations Another possible parametrization is the following one: Constraints inside the bins are correlated. With current data, an increase in the number of bins does not change the result. BSH is BAO, SN-Ia and Hubble constant constraint.

12 Quintessence But do we have physical models different from a cosmological constant that can lead to an accelerated universe? If we consider a scalar field minimally coupled to gravity the action can be written as: where and is the field potential. Varying the action respect to the field we have the equation of motion:

13 Quintessence The energy momentum tensor can be written as: Energy and pressure densities of the field are given by: Leading to the Friedmann equations:

14 Quintessence The equation of state can be written as: Note: w is always larger than -1! We therefore have an accelerating universe (w<-1/3) if And we expect a time-evolving equation of state! Example: And we have acceleration with p>1

15 Quintessence Tracking Most of these models show a tracking behaviour.

16 Quintessence Several model of quintessence (and even of modified gravity as DGP) are well mapped by the CPL parametrization. The current models of quintessence that provide the best fit to observations are of the type of thawing quintessence. These models have w=-1 at high redshifts. For thawing models actually one parameter is enough, fixing:

17 Quintessence - There are plenty of Quintessence models. - Quintessence tracks the dominant energy component, this helps in solving the why now problem. - The transition to an accelerating universe is often connected to the radiation-matter equality. - Problems with Quintessence: energy scale too low, long range forces not observed.

18 Modified Gravity On the other hand, one could consider cosmic acceleration as a failure of General Relativity at cosmic scales. One possibility to modify gravity is to include a function of the Ricci Scalar in the action: New term Energy Content: ordinary matter!

19 Modified Gravity - f(r) This brings to new Friedmann equations: In practice, there are 2 workable f(r) models: Hu and Sawicki Hu W., Sawicki I., 2007, arxiv: v1 Starobinski Starobinsky A.A., 2007, JETP Lett., 86, 157 When compared with observations, the best fit parameters of the models produce an acceleration very close to lambda.

20 Hu-Sawicki If fitted as a dark energy component, the Hu-Sawicki model provides an equation of state that varies with redshift and crosses w=-1. Current constraints on this model are weak.

21 Angular Diameter Distance We can measure the distance of an object by measuring its angular size and knowing its size (standard ruler). In the comoving reference frame we have: The angular diameter distance of an object at redshift z is:

22 Angular Diameter Distance In cosmology, the angular diameter distance and the luminosity distance of the same object can be completely different!

23 Angular Diameter Distance:SZ+X ray clusters The hot gas in a cluster of galaxies produces a distortion in the blackbody spectrum of the Cosmic Microwave Background that is frequency dependent. (Inverse compton scattering, photons are shifted to higher energies).

24 SZ Effect in CMB maps Abel GHz 217 GHz 70 GHz 100 GHz 353 GHz 143 GHz 545 GHz

25 X ray emission from Clusters Cluster of galaxies also emit X-ray radiation due to bremsstrahlung of ionized hot ( megakelvins) intracluster gas

26 Angular diameter distance To put it simply we have that: SZ: absorption X-ray: emission Integral over the cluster volume Free electrons density If the cluster is almost spherical we have: By measuring absorption and emission we measure the size of the cluster and we can get its angular distance!

27 Angular distance from clusters Useful for measuring the Hubble constant. Bonamente et al.,

28 Etherington s distance duality In principle, we can use standard candles and standard rulers at the same redshift to test this relation. It is a fundamental prediction of an expanding universe.

29 Test of distance duality Assuming eta as a constant:

30 Lookback time The time that a photon emitted at redshift z has spent to reach us is given by (omitting radiation): This time is clearly the difference between the age of the universe minus the age of the object that sent the photon and the age of the universe at the redshift of formation of the object: Age of the universe Age of the object Age of the Universe at z of object s formation

31 H(z) from cosmic chronometers The Hubble parameter depends on the differential age of the universe in function of redshift. Differential redshift Differential Age If we measure the age and redshift of different objects for close enough redshifts and ages we could estimate the derivative and so H(z).

32 H(z) from ages Left Panel: age of passively evolving galaxies obtained from stellar population synthesis models in function of z. Right Panel: H(z) obtained from differential ages from the same catalog. See

33 Constraints on w Open: just CMB Filled: CMB+H(z) (from cluster ages)