1. Theoretical Perspectives on Cosmology and Cosmic Dawn Scott Dodelson: Science Futures in the 2020s

2. U NIFORM TO ONE PART IN 10,000 V ERY NON - UNIFORM This evolution was driven by gravity: over-dense regions became more over-dense, eventually forming galaxies and stars For thousands of years, we have tried to understand our place in the Universe

3. We are about to hear exciting ideas for how to fill in the gaps and details: How did galaxies form? When and how was the universe re-ionized? What was it like before then? What was responsible for the initial perturbations? For thousands of years, we have tried to understand our place in the Universe: How did we get here?

4. Another age-old question is: what are we made of? Over the past century, physicists have made immense progress understanding the basic building blocks of nature Atoms: nucleons and electrons Nuclei: proton and neutrons Quarks and gluons Heavy force mediators: W and Z Heaviest quark (top) The Higgs! New physics?

5. The two questions are intertwined The evolution of the Universe cannot be understood with just this physics What are the new forces and new particles that are required to explain the universe and how do they fit in to the rest of physics?

6. We have ideas … Dark Energy: The energy at these low scales is dilute but everywhere, so that the amount currently spread uniformly throughout the Solar System is equivalent to the total energy that will be used on Earth over the next trillion years. Inflation: The energy up here was highly concentrated, so even a microscopic speck contains much more energy than exists in the Solar System Cosmic surveys have provided evidence for physics at very high and very low energies Neutrinos: With masses 12 orders of magnitude smaller than all other particles, understanding the pattern of neutrino masses and identifying how many there are may open the door to new physics

7. Inflation: It’s not just gravitational waves f NL =-100 f NL =+100 Lidz, Baxter, Adshead, Dodelsoln 2013 f NL =+300 f NL =0 Measure Primordial Non- Gaussianity with galaxy or H surveys

8. Neutrinos Inhibit Small Scale Structure Neutrinos have large thermal velocities so do not cluster on small scales. This non-clustering component inhibits the formation of small scale structure. Massless Neutrinos Massive Neutrinos

9. Neutrinos Inhibit Small Scale Structure We quantify this with the power spectrum, or the dimensionless Δ 2 = k 3 P(k)/2π 2.

10. Current upper limits at ~0.2 eV CMASS: Zhao et al. 2013

11. Future Surveys should do better by a factor of 10 0.016 CMB S4 (+DESI BAO) DESI (+Planck) 0.017 LSST (+Planck) 0.023 Lower bound from oscillation experiments of 0.05 eV

12. CMB Anisotropies: Ratio of Acoustic Scale to Damping scale The ratio of the damping scale to the acoustic scale:

13. Neutrinos affect this ratio Einstein’s equations  Expansion Rate proportional to energy density Age of the universe at given Temperature is higher if expansion rate is slower (e.g., if neutrinos did not contribute to the energy budget) More neutrinos  more damping

14. More neutrinos  More Damping  Less power on small scales

15. More than 3 light neutrinos Disfavored Number of Cosmologically Produced Neutrino Species Black contours use more data sets (TT, TE, EE, BAO): N=4 excluded at 5-sigma “leads to the robust conclusion that ΔN eff < 1 at over 3-sigma.” 1502.01589

16. Future  percent level Manzotti, Dodelson & Park 2015 Test our knowledge of the universe when it was 1 sec old It’s not just neutrinos: Many models with hidden sectors predict particles that were in equilibrium early on, were diluted, and now remain at the percent level Need CMB-S4+ other experiments

17. Plan for Dark Energy: 4 Independent probes plus uncorrelated CMB  Constraints on w (and w’) Supernova Brightness Baryon Acoustic Oscillations Gravitational Lensing Galaxy Cluster Abundance Cosmic Microwave Background

18. Plan for Dark Energy: 4 Independent probes plus uncorrelated CMB  Constraints on w (and w’) Supernova Brightness Baryon Acoustic Oscillations Gravitational Lensing Galaxy Cluster Abundance Cosmic Microwave Background This framework is breaking: Acceleration can be explained without dark energy CMB is correlated with other probes Four probes are correlated with one another Upshot: We are just beginning to understand how to extract information about the physical mechanism driving the current acceleration

19. Acceleration without dark energy Can explain acceleration without dark energy (w) by modifying General Relativity: For the cosmological metric, the acceleration equation generalizes to: Get acceleration if these terms are positive

20. Easy to fit Expansion History Santos et al. (2008)

21. Our “Neptune”: Growth differs in Modified Gravity models because they introduce new degrees of freedom Projections for DESI Glossary: f(R) – modified gravity model DGP – modified gravity model ΛCDM – dark energy mode, with DE=cosmological constant DESI – Dark Energy Spectroscopic Instrument, big survey to start in 2018 Snowmass: year-long process, during which high energy physics community made science case for different projects Snowmass: Huterer et al. 2014

22. Dark Energy Survey Galaxies X South Pole Telescope Lensing (Giannantonio et al 2015)

23. Cross-correlation in each redshift slice picks out contribution to CMB convergence from that slice Can infer growth of structure

24. DES Galaxies X SPT Lensing First estimate of growth function … early days (Giannantonio et al 2015)

25. The future is bright: there is a lot more information in the sky We have surveyed a fraction of the observable universe equal to the fraction of the globe covered by …

26. The future is bright: there is a lot more information in the sky We have surveyed a fraction of the observable universe equal to the fraction of the globe covered by New Jersey

27. Consider the United States in 1790  Over-densities of order 50  Concentrated in East  Vast Voids with low density

28. Consider the United States Today  Over-densities of order 10,000  Concentration in coasts  Traces of primordial density (Boston- Washington; East > West)  Vast Voids

29. The story of this evolution is the story of the United States If we understood all the forces – sociological, economic, racial, and political -- acting on the constituents … we should be able to explain these patterns and this evolution