anghara (anghara) wrote,
anghara
anghara

Launchpad Day 5, PM, Part the First - The Fate of the Universe

We were talking Cosmology.

These notes will probably have to be updated and rehashed later as I play catch-up - because by
this stage the astronomy train was moving so fast that I barely had time to grab any notes at all if I wanted to keep listening to what was coming next - and everything that was coming next seemed just as important if not more so than the presentation slide that just slipped by without my having time to catch more than a couple of keywords off it. Astronomy 101 in a week is a lot of fun but it's *hard work*.

We did start out with two URLs - Ned Wright's cosmology page, with sections on tutorials with lots of answers lots of questions and Java simulations and a cosmology calculator, and Wayne Hu's page, with more tutorials and FAQs, and I suspect I'll have to return there at some point before this all comes completely clear to me.



On large scales, galaxies are moving apart with velocity proportional to distance (Hubble's Law) – but it isn’t the galaxies themseles moving through space: space is expanding carrying the galaxies along, the galaxies themselves are not expanding. In terms of galaxy formation, on small scales gravity always wins - until some point the gravity and the expansion forces get balanced out and after that point the expansion gains the upper hand. This is why galaxies themselves don't fall apart. We are not, alas, probably much to some people chagrin, the center of the universe. If you were standing at any other point in the known universe you would get to see much the same cosmological picture of an expanding universe.

It is possible to estimate the age of the universe: knowing current rate of expansion we can estimate the time it took for galaxies to move as far apart as they are today – age of universe works out to about 14 billion years. But the rates of expansion have NOT been constant throughout time, so these 14 billion years are... wait for it... an approximation. The point is that there is an observable universe - looking further and further away we see further and further back into time - and the limit to this observation is about 14 billion years. There is a reason for this limit, which we will get to shortly - let us just assume for now that there might be a lot of universe BEYOND those 14 billion years which are unable to observe.

The radiation from the very early phase of the universe should be detectable today – and indeed was discovered in 1960s as cosmic microwave background radiation, which is basically black body radiation with a temperature of 2.73 K. This is a picture of an almost perfect black body, radiation wave length of about 0.1 cm (3000 K blackbody redshifted by a factor of 1000 – 14 billion LY away)




...or, the reason for those 14 billion years.

For purposes of clarity the [bracket] numbers imply superscript, I didn't have time to make pretty at the time I might come back and fix this later.

You can do a plot of time versus temperature which looks at the Universe from the big bang to the present day.

Temp going DOWN from 10[10]K (early stages of formation of helium) to 0 K; the universe is cooling down as time passes

In terms of time,from 1s to about 10[4] years post-Bang, radiation dominates in the Universe; from 10[4] to 10[10] years (about right now), matter dominates. Energy density=matter energy equilibrium at about 10[5]-ish years.

A "recombination" event occurs just before 10[6] years on this plot, and a "reionization" event (first stars and galaxies) at roughly 10[9] years. Early universe was very high energy, and opaque to photons and therefore our sight. In a nutshell, there was a soup (don't look at me that was the word they used) of a bunch of electron positron pairs, merrily recombining into energy. Electrons, positrons and gamma rays were in equilibrium between pair production and annihilation. From the soup, things cool off to the point where protons and neutrons form a few Helium nuclei; the rest of the protons remain as Hydrogen nuclei (isolated neutrons are unstable, with a half life of about 15 minutes, so they don't hang around). Some of these molecules stick together and are stable enough not to dissociate immediately on gamma ray impact. So - RECOMBINATION EVENT – protons and electrons recombine to form atoms, universe becomes transparent for photons, universe a few hundred thousand years old at this point. Neutral hydrogen makes the universe transparent to photons. This leads into the formation of elements ladder - fusion building up larger and heavier elements in turn; the entire universe is the heart of a star at this point - but there is still a uniform distribution of this stuff. We can "look back in time" to the recombination event but from our POV anything before the energy/matter equilibrium looks exactly like the surface of a star, bright and opaque (still ionized). At this point the mass distribution is approximately 25% in helium, 75% in hydrogen.

After recombination z=1000 (red shift), t=3000K photons can travel freely through space – 3000K black body cosmic background microwave radiation.

What happens next, well, photons are incessantly scattered by free electrons; photons are in equilibrium with matter – radiation dominated era. After the recombination, there was a reionizing event where gas got reionised (z (red shift) = 16 to 6) After less than 1 billion years the first stars form – UV light from first stars re ionizes gas in early universe.




Certain fundamental assumptions have to be made. They include homogeneity (on largest scales the local universe has same physical properties throughout the universe - every region has the same physical properties, mass density, expansion rate, etc.), isotropy (the concept of which I kind of get but I need to go and get the precise definition somewhere because I missed it on the slide) and universality laws of physics same throughout the universe) I really need to go back over this particular section and fill in the blanks properly. But by this time we were roaring down this material at half the speed of light and it was all I could do just to keep up...

Space time tells matter how to move; matter tells space-time how to curve.

Effects of gravity on the largest cosmological scales should be related to curvature of space=time which is in turn determined by distribution of mass and energy in universe




Expansion should be slowed by mutual gravitational attraction of the galaxies.

Fate of universe depends on the matter density of the universe.

Define a “critical density”, which is just enough to slow the cosmic expansion to a halt at infinity

If density of matter equalled the critical density, then the curvature of space would be just so sufficient to make the geometry of the universe flat. If density smaller than critical density universe expands forever – open parabolic curvature – no gravity. At density greater than critical, universe will collapse back under influence of gravity – circular geometry – closed universe.

Problems with classical decelerating universe:

1 the flatness problem universe seems to be nearly flat even a tiny deviation from perfect flatness at the time of the big bang should have been amplified to a huge deviation today => extreme fine tuning required!

2 isotropy of cosmic background – if information can only travel at speed of light then structure in the cosmic background should not be correlated over large angular scales => contradiction to almost perfect isotropy of the cosmic background (look up definition of the word isotropy)

Where there's a problem, 21st century cosmology has the solution – inflation! Expanding faster than the speed of light!

Inflation = period of sudden expansion during the very early evolution of the universe triggered by the sudden energy release from the decoupling of the strong and electroweak forces - very like a balloon expanded very suddenly from nothing to very large – outside surfaces then LOOK flat (like earth) even though it is actually a very large sphere.

So they set about measuring the "deceleration" of the expanding universe, by observing Type Ia supernovae and measuring Hubble relation at large distances. Odd things happened.

Distance <-> recession speed

Size scale of the universe> rate of expansion

And we found that the expansion of the universe… is accelerating instead of decelerating

So things become more complicated.

Cosmological constant – cosmic acceleration can be explained with the cosmological contstant, [capital lambda]

A free parameter in Einstein’s fundamental equation of general relativity previously believed to be 0

Energy corresponding to [lambda] can account for the missing mass/energy needed to produce a flat space-time = “dark energy”

Unh. yeah. another slide I mostly missed. Need to go back and noodle this out properly.)

But anyway, where were we - the evolution and ultimate fate of the universe.

Big bang = deceleration = at some point dark energy overcame gravity, and acceleration began about 6 billion years ago. This new information leads to new and different models of the universe, known as the Big Empty (everything moves away at >c and eventually we can't see anything any more except the local cluster - galactic-scale astronomy *goes away*) and the Big Rip (acceleration eventually is so great that it tears apart the atoms themselves and all is destroyed). (Have to go back and get perfect definitions of those - the whole thing is beyond fascinating)

Large scale structure: formed from gravitational collapse (and looks rather like pumpkin innards in pictures)

A large survey of distant galaxies shows the largest structures in the universe: Sloan Great Wall. As for the rest - filaments and walls of galaxy superclusters, voids, basically empty space.

Angular size of cosmic microwave background allows us to probe the geometry of space time – we can model what we would see in different models but what we see corresponds most closely to the geometry of a near-flat universe (within a couple of percent)

Dark energy = 70% of universe


There you have it. The fate of the universe in a single blog entry. Dark energy, indeed.

I'll have to go over this session a bit more meticulously later to fill in gaps left by slides that whipped on and whipped off far too quickly, but this is the bare-bones version of it. And I'm still thinking about it all.
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