The Accelerating Universe

1. The Accelerating Universe • Megan Donahue • Michigan State University

2. The Accelerating Universe • What is the evidence that the Universe is not only expanding, but accelerating? • What is dark energy? • What does an accelerating Universe imply?

3. Edwin Hubble (1889-1953) • Found that galaxies (outside the Local Group) are ALL receding away from us. • The most distant galaxies appear to be receding the fastest. • Note: this pattern only applies to galaxies.

4. Hubble’s “Law” for Galaxies The slope is the Hubble’s constant Relates the recessional velocity (or redshift) to distance Implies that the universe is expanding (from everywhere)

5. Visualization aids for space that expands • Rubber Band Model with paper clips (1-D) • Balloon Model with stickers (2-D) • Animations

6. Implications of an expanding universe • The farther the galaxy, the faster it appears to be receding from us (the larger its redshift.) • At earlier times, the galaxies were closer together. • We can “run the movie backwards” to get the age of the universe: the faster the expansion, the younger the inferred age.

7. Newton and Einstein • Newton knew an infinite universe was unstable. • Newton was uncomfortable that his theory of gravitation invoked “action at a distance”. • Einstein’s General Theory of Relativity solved these problems...

8. Einstein’s Theory of Gravity • Mass causes spacetime to curve. • Light takes the shortest path through spacetime - which may be curved. • Gravity is not instant action at a distance: gravitational changes propagate at the speed of light. • A static, unchanging universe is unstable.

9. Einstein’s “Greatest Blunder” • Einstein grew up believing the universe was infinite and static. • He added a mathematically-allowed constant to his solutions: The Cosmological Constant. • He could have predicted that the universe was either expanding or contracting: he called this his “greatest blunder.” [Second-hand report from the autobiography of George Gamow.]

10. The Expanding Universe • How fast is the universe expanding? (What is the Hubble’s Constant = the slope of the line in the Hubble diagram?) • How quickly is the expansion slowing down?

11. The Hubble Space Telescope (HST) • The distances to galaxies were most accurately measured by studies with HST. • Within 10%, the Hubble constant is ~70 km/sec/Megaparsec.

12. One small problem... • The age of the universe with a Hubble’s constant of 70 km/sec/Mpc including the expected slowing of the expansion due to matter was uncomfortably close to the age of the oldest stars (10 billion years).

13. Expectations • The Hubble constant isn’t really constant - gravity should have been slowing down the expansion of space over cosmic time. • To detect the changes with cosmic time, one needs to measure redshifts and independent distances for distant galaxies whose light was emitted a long time ago.

14. Cosmic Standard Candles • “Type Ia” supernovae are bright enough to be seen everywhere in the observable universe. • This type of supernova is a complete explosion of a white dwarf that had grown too massive to support its own weight: all white dwarfs will blow up if they exceed 1.4 times the mass of the Sun.

17. Supernova Observations • Light curves measure peak brightness. • Spectra measure redshift (recession velocity) and confirm type of supernova.

19. The Universe is...what? • By 1998, supernova researchers knew something was afoot: the data showed the expansion of space was NOT slowing down but SPEEDING UP. • The data since then have strengthened this result into a fact: the universe is not only expanding, it is accelerating.

20. Maybe we should have known... • The age of an accelerating universe is older, making what we know about stellar evolution consistent with HST’s Hubble constant. • The combination of cosmic microwave background results with our results (including MSU astronomers) from clusters of galaxies implies an accelerating universe.

21. What causes the universe to accelerate? • Gravity causes matter to only pull = slow down (not accelerate). • But Einstein’s theory of gravity does allow for repulsion IF the cosmological constant is positive.

22. The cosmological constant • A vacuum is not perfectly empty. • Quantum physics predicts that the vacuum is full of particles and anti-particles that pop into existence and a short time later, annihilate each other. • Therefore even a “perfect” vacuum has an energy density that would behave exactly like a cosmological constant.

23. The cosmological constant • Theoretical estimates of this constant predict that it should be HUGE. (10120) • Astronomical observations showed it had to be small (<1-2), and so prior to 1998, most assumed it was zero, that something missing from the theory cancelled it exactly. • Today’s cosmology experiments all seem to point to a value of 0.7!

24. Dark Energy • “It’s got a cool name.” • It’s not matter. • We can’t see it. But its effects show it comprises about 70% of the mass-energy budget of the universe.

25. Dark Energy • It could be the vacuum energy (a cosmological constant). • It could be something else.

26. What’s next? • How dark energy behaved in the past may give clues about its nature. • We astronomers have the biggest lab: the universe. :) • Large projects to do cosmology, for better statistical confidence and the exploration of systematic uncertainties, are planned and underway: the next 2 decades will see greater precision.

27. What’s next? • And maybe a few more surprises.

28. Spartan Cosmology • Generation and analysis of supercomputer simulations of the universe (O’Shea, Voit) • Statistical analysis of cosmological observational data (Voit, Donahue) • Theory and observational studies of the first stars and supernovae (O’Shea, Beers) • Physics of white dwarf supernovae (Brown) • Supernova studies with MSU’s Chilean telescope, SOAR (Loh) • Evolution of clusters of galaxies and of galaxies (Donahue)

29. Cluster in Visible Light • MS0451.6-0305 • RGB colors • Clowe, Luppino, Kaiser, & Gioia 2000

30. Cluster in X-ray Light • View from the Chandra X-ray telescope • Donahue et al. 2003

31. Clusters of galaxies • Most massive, gravitationally-bound structures in the universe (~1015 x Sun’s mass) • Hot gas outweighs the stars by a factor ~ 6 MS1054-0321 Donahue et al 1998

32. Weighing clusters • Cluster masses can be measured independently: • Galaxy velocities and positions • Gas temperature and cluster size • Gravitational lensing (Donahue 1996; Donahue et al 1998)

33. Clusters of galaxies • Most massive, gravitationally-bound structures in the universe (~1015 x Sun’s mass) • Hot gas outweighs the stars by a factor ~6-10 • Dark matter outweighs the baryons by a factor ~ 8.

34. Clusters and Cosmology • Clusters form as the result of gravitational attraction. • The seeds of formation are the initial fluctuations of density in the early universe. • The formation rate (the “growth function”) is sensitive to the overall density of the universe (Omega_matter)

35. Low Density High Density Borgani & Guzzo, 2001, Nature.

36. ConcordanceWM-WL Dark Energy Vikhlinin et al 2003 Dark Matter

37. Model predictions for the number of clusters • M > 1 x 1015 solar masses

38. Systematics • Selection Effects: what gets counted, what gets missed? • Evolution of cluster mass surrogates: • X-ray luminosity • Baryon fraction • X-ray temperature • K-band light (infrared light from stars)

39. Mass -Temperature Relation X-ray temperature is estimated from fits of emission models to the X-ray spectrum

40. Luminosity - Temperature Relation Luminosity requires fewer X-ray photons to measure than Temperature

41. To improve our bounds on cosmological models with X-ray-based studies of clusters, what do we need to do better? • To understand the physics well enough to predict the evolution: the astrophysics of the intracluster gas. • More observations to better describe the statistics and evolution of clusters.

42. Summary • The universe is not only expanding, it is accelerating. • The first direct evidence for acceleration came from studies of white-dwarf supernovae over a vast range of distances from our Galaxy. • Other cosmological studies (including those conducted at MSU) are all (so far) consistent with this picture - we all get the same value for the dark energy density. • Larger studies, with smaller uncertainties, will allow us to learn more about this “dark energy”, and provide more clues for the theorists working on the dark energy question.