🗊Презентация Today in Astronomy 102: the age, and fate, of the Universe

Today in Astronomy 102: the age, and fate, of the Universe Matter-dominated universes, and measurements of the mass density of the Universe: an open Universe? Direct measurements of the Universe’s curvature: a flat Universe? Time without end: the Universe does not appear to be a black hole, is probably open, and will probably expand forever.

From last time: the cosmic microwave background is almost too isotropic. One theoretically-popular way out of this problem is to postulate a brief period of inflation early in the Universe’s history. Briefly, this is thought to happen as follows. Shortly after the Big Bang, the vacuum could have had a much larger energy density, in the form of virtual pairs, than it does today. This possibility is allowed under certain theoretical models of numbers and interactions of elementary particles. At some time during the expansion, the vacuum underwent a phase transition (like freezing or condensing) to produce the lower-energy version we have today.

Inflation: the cosmic microwave background is almost too isotropic (continued). While the vacuum was in its high-energy-density state, it gave a large additional impulse to Universal expansion. Recall: vacuum fluctuation energy density is actually negative in strongly curved spacetime; virtual pairs were exotic in the newborn Universe. Thus the vacuum acts “anti-gravitationally” early in the expansion. Accounting for the vacuum’s influence in general relativity leads to a very much smoother and faster expansion. During this period, spacetime’s radius of curvature increases more like a bubble blowing up, than like a blast wave - hence the name inflation for the process. During inflation, the vacuum would appear in the field equations as a cosmological constant.

Inflation: the cosmic microwave background is almost too isotropic (continued). The inflationary era would have been relatively brief, much shorter than the time between Big Bang and decoupling. If it lasted through 100 doublings of the Universe’s size, that would do it, and this takes only about 10-35 seconds. During the remaining “normal” expansion between the end of inflation (decay of the vacuum to its low energy density state) and decoupling, the bumps and wiggles normally present in blast waves still wouldn’t have had enough time to develop. We know of course that the Universe has become much less smooth since decoupling. The seeds for inhomogeneities like galaxies, stars and people were not sown before decoupling, however.

The age and fate of the Universe The expanding Universe resembles the interior of a black hole. Is the Universe a black hole? That is, is the universe open, marginal, or closed? If it’s not open, it really can be thought of as a black hole. Related question: How old is the Universe? That is, how long has it been since the expansion (and time) began? If the Universe’s total energy is matter-dominated (that is, if the cosmological constant is zero), the age, expansion rate, curvature and fate all turn out to be determined by one factor: how much density (mass per unit volume) there is in the Universe. We usually illustrate this by general-relativistic calculation of the typical distance between galaxies as a function of time elapsed since the present day…

The age and fate of the Universe (continued)

The age and fate of the Universe (continued)

How can we tell which “universe” is our Universe? Several ways are possible, all with substantial and different degrees of difficulty: Measure the density directly, using observations of the motions of galaxies to determine how much gravity they experience. (Much like our way of measuring black-hole masses by seeing the orbital motion of companion stars.) Measure the ages of the oldest objects in the Universe. Measure the Universe’s curvature directly, by observing very distant objects with well-determined size and distance. Measure the acceleration or deceleration of galaxies: the rate of change of the Hubble “constant.” The first two ways are least difficult and provide most of our data. In order…

1. Is the Universe gravitationally bound? Matter-dominated universes. If the Universe is dense enough at present, the mutual gravity of its parts will eventually result in a slowing or reversal of the expansion. The density that would make the Universe marginal can be calculated from general relativity and is

1. Is the Universe gravitationally bound? Matter-dominated universes (continued). So what is the present-day density of the Universe? Observational bounds on m, made from “nearby” galaxy redshift surveys over the past 15-20 years, consistently indicate that Right: summary of measurements of the Universe’s mass density (N. Bahcall 1997)

1. Is the Universe gravitationally bound? Matter-dominated universes (continued). So if the Universe is matter-dominated, its curvature is negative, it is open, and it will continue to expand. It is, however, a strong theoretical prediction many models of elementary particles and of the early Universe, especially those involving inflation, that m should be exactly 1, and that for unknown reasons the present measurements of m are faulty. Observers and theoreticians used to argue incessantly about this. There are no good experimental results or theoretical arguments to suggest that the universe is matter-dominated and closed. We don’t think our Universe is a black hole.

Mid-lecture break Homework #7 is due on Friday at 11 PM. Exam #3 takes place Thursday, 20 December 2001, 4-5:15 PM, right here. The TAs are scheduling a review session: stay tuned to your e-mail. Don’t forget the practice exam, available on the AST 102 Web site.

2. Age of matter-dominated universes General relativity can be used to show that the age of a matter-dominated universe is always given, in terms of the present value of the Hubble “constant”, as where the value of the factor A depends on m, but is less than or equal to 1. The factor A is equal to 1 if m is very small compared to 1. The larger the value of m, the smaller the value of A. Open universes have values of A between 2/3 and 1, and closed universes have values of A smaller than 2/3. Jargon: t = 1/H0 is often called “one Hubble time.”

2. Age of matter-dominated universes (continued) If m is assumed to be much smaller than 1, the age would be If m is assumed to be 1, the factor A turns out to be exactly 2/3, and the age is

2. Age of matter-dominated universes (continued) For the best experimental value, m = 0.2, we get Other constraints on the Universe’s age, independent of density determinations: We know that the Universe must be older than the solar system, which is 4.5109 years old, so an age of 1.31010 years would be OK on this score. The ages of white dwarf stars and globular star clusters turn out to be accurately measurable; the oldest of these are 1.31010 years old (± about 0.11010 years). This agrees with m = 0.2 (smaller would be OK too), and is in conflict with m = 1.

2. Age of matter-dominated universes (concluded)

3. Measurements of the Universe’s spacetime curvature: is it not matter-dominated? The very small fluctuations in the cosmic microwave background – a.k.a. the background anisotropies – provide the means to measure the curvature of the Universe rather directly. Reasons: Before decoupling, the Universe consisted of ionized gas in equilibrium with photons. This gas-photon mixture took the form of bubbles with very slightly different densities and temperatures. If a bubble were compressed by its neighbors, it heated up and pushed back on its neighbors all the harder. Thus the bubbles could oscillate in size and temperature. The cosmic microwave background is a snapshot of the final state of these bubbles, and the anisotropies outline the bubbles.

3. Measurements of the Universe’s spacetime curvature: is it not matter-dominated? (continued) It turns out that the bubbles that are the most numerous are the ones that have only gone through half an oscillation between the Big Bang and decoupling. Their diameters can be calculated precisely. By observing their angular size and knowing their diameters we can determine the curvature of spacetime between decoupling and here-and-now.

3. Measurements of the Universe’s spacetime curvature: is it not matter-dominated? (continued) Detection of cosmic background anisotropies on the scale of these bubbles has become possible in the last few years, in high-altitude balloon-borne measurements by the MAXIMA and BOOMERANG instruments.

3. Measurements of the Universe’s spacetime curvature: is it not matter-dominated? (continued) Result: the curvature between decoupling and here/now is zero – a flat Universe!

3. Measurements of the Universe’s spacetime curvature: is it not matter-dominated? (continued) If these results are true: how did the Universe come to be flat? We know that m = 0.2: there isn’t enough matter in the Universe to make it flat. There aren’t enough photons, either. What’s left? The easiest way out seems to be a positive cosmological constant. (See lecture, 4 December 2001.) For the cosmological constant  one can define a relative “density” . For the Universe to be flat, m +  = 1. But m = 0.2, so  = 0.8; the cosmological constant dominates the Universe’s present mass-energy density on large scales. If there is an afterlife from which we can be seen, Einstein is having a really good laugh about this.

3. Measurements of the Universe’s spacetime curvature: is it not matter-dominated? (continued) This changes everything. If the cosmological constant is nonzero, then there is no longer a one-to-one correspondence between curvature, boundedness and fate. For example: If the value of  were negative, the universe would collapse and end in a singularity no matter what its curvature. If the value of  were positive and large, even a positively-curved, closed universe would expand forever. (4.) If  = 0.8, distant galaxies should be seen to accelerate. This may have been confirmed, recently, in observations of distant galaxies in which supernovae have been seen.

Age and fate of the Universe if it has a positive cosmological constant

Age and fate of the Universe if it has a positive cosmological constant (continued)

Summary: best (experimental) determination of the state of the Universe The Universe has a present-day relative mass density of about m = 0.2. If matter dominates its energy, the Universe is negatively-curved and open, the presently-observed expansion will continue forever, and about 1.3x1010 years (13 billion years) have elapsed since the Big Bang. There are indications, in experiments which need to be reproduced, that the Universe is flat. This requires a substantial, positive cosmological constant, which dominates the present energy of the Universe:  = 0.8. (It also requires a physical explanation for !) If this is true, the Universe is open, the present expansion will continue and will increase dramatically over time, and the Universe is about 1.6x1010 years (16 billion years) old. The NASA MAP satellite, launched this year and just beginning its mission, will settle the cosmological-constant issue once and for all. Stay tuned; the final answer may appear in the next few years.