THE NEW COSMOLOGY
The history of the universe in a nutshell
The Big Bang ( t = 0 )
This is the moment of creation of space and time. We think the universe was spontaneously birthed from virtual energy in a vacuum. Such vacuum energy is known to exist, and may provide clues to the nature of dark energy.
As the universe evolves, it expands, cools, and the density decreases at it expands. At this early stage in the universe, the expansion slows down with time (deceleration) as the self-gravity of all the matter in the universe is pulling on itself.
Nucleosynthesis ( t = 3 mins )
The conditions in the universe are now cooled enough that the universe is similar to the conditions in the core of a very hot star. The temperature is a few billion degrees Kelvin. The gas is a plamsa (free nuclei and electrons). The conditions are just right for a few minutes for hydrogen to fuse into helium. By the time the unievrse has expanded and cooled so that the fusion stops, the mass of the two elements elements of the universe are 1/4 helium nuclei and 3/4 hydrogen nuclei. Deuterium and lithium are also fused, but very little of it. But the amount of deuterium is very sensitive to the average density of the universe and today we use the deuterium and lithium abundances as an important constraint on the portion of the total density contributed by "normal" matter. See more below. At this stage in the universe, the expansion is still slowing down with time (deceleration).
Recombination ( t = 380,000 yrs ) and origin of the cosmic background radiation
The universe has now cooled so it is similar to the conditions in the atmosphere of a star, like the sun. The temperature is a few thousand degress Kelvin. Now, free electrons are moving slowly enough that they get captured by the hydrogen and helium nuclei for the first time. This marks the first time simple atoms (with bound electrons) are formed. As the electrons are captured, they emit ultraviolet light (like in an emission spectrum). Thus, there is a big burst of light in the universe. Today, we can measure this burst of light as the cosmic microwave background (called the CMB). The reason we observe it in the microwave region of the spectrum today is because the ultraviolet light has been cosmologically redshifted (the wavelength of the original ultrviolet light has been stretched) from the expansion of the universe from t=380,000 years to the present. At this stage, the expansion is still slowing down with time (deceleration).
The Dark Ages ( t = 380,000 yrs to t = 1 billion yrs )
Following recombination until the first stars start to form, the universe is totally dark. This is known as the Dark Ages. This is also sometimes called the Era of Atoms. The universe continues to expand and cool and its density continues to decrease. At this time, the expansion is still slowing down with time (deceleration).
Structure Formation ( t = 1 billion yrs )
Through gravitational attraction, gas contracted into the first stars and then into galaxies. The first star light lights up the universe. As time progresses to the present, galaxies and galaxy clusters form and evolve. At this time, the expansion is still slowing down with time (deceleration) as the self-gravity of all the matter in the universe is pulling on itself.
Acceleration ( t = 6 billion yrs )
Until now, the so-called dark energy was too weak to make an effect. The story of dark energy is told below. At this time, however, the dark energy, which tends to act like anti-gravity, begins to dominate over the gravity. The dark energy causes the expansion of the universe to pick up speed again. This is known as acceleration. So at this time, the expansion of the universe is no longer slowing down, but speeding up (accelerating).
Present Day ( t = 14 billion yrs )
What you see is what we got. Galaxies are fully formed, stars are born and die in life cycles, generating more complex atoms (carbon, nitrogen, oxygen, iron, etc). The galaxies live in groups, clusters, and rich clusters. The matter is spread out over a complex topology that includes sheets, filaments, and voids. Galaxies are clustered along the filaments, and rich clusters reside at the intersection of filaments.
GEOMETRY OF THE UNIVERSE
There are three possible geomoetric shapes to the universe. We call them open, flat, and closed. Open geometry has a saddle-like shape. Closed geometry has a spherical-like shape. Flat is flat. Which geometry the universe is depends on the average density of the universe. We define a critical density at which we computed that the universe would be "flat". If the density is less than the critical density, then the universe would be "open". If the density is greater than the critical density, then the universe would be "closed".
FUTURE AND FATE OF THE UNIVERSE
Old school of throught pre-1990s
Since the rate at which the expansion of the universe slows down (decelerates) depends upon the amount of gravity pulling the expansion back, we know that the density of the universe is very important. If the density is less than the critical density so that the universe is "open", then it will expand forever. If the density is greater than the critical density so that the universe is "closed", then universal expansion would reach a maximum, stop, and then turn around in a contraction (called the big crunch).
Summary of the Fate of The Universe: (old school of thought)
density of universe geometry fate (old school) ------------------------ ----------- ---------------------------- density = critical value FLAT expansion eventually "freezes" density > critical value OPEN expansion goes forever density < critical value CLOSED expansion maximizes, then stops, then contracts on itself
New school of throught early 2000s
So, is the universe flat, open, or closed? We did not know until about the year 2000. It is flat. This is where it gets confusing. We have recently learned that the universe is flat, but even more recently learned new information that tells us that it is actually NOT related to the matter density (per the old school of thought above). And we have also recently learned that the fate of the universe is not dependent upon its geometry (per the old school of thought)!!! So what is the story? First, why do we know the universe is flat. This comes from observations of the cosmic background radiation (CMB, which arose from the event called recombination). Temperature variations in the CMD ara about a degree in angular size. This is the size we expect if the universe is flat. If the universe is open, the spots would be smaller; if the universe is closed then the spots would be larger. To great precision, we know the universe is flat.
BUT, we know that the density of matter is only 4% of the crtical value! We know this because the relative abundances of deuterium and lithium, created when the universe was 3 minutes old, is very sensitive to the density of matter. It turns out that the higher the density, the less deuterium is formed. We have observed that much more deuterium was formed than would have been if the density of the universe had its critical value. The density of matter is 4% of the critical value for the measured abundance of deuterium! Where it the other 96% of the critical density?
Dark matter is another form of matter. We know it exists for two reasons: (1) the rotation speeds of galaxies are so fast that galaxies should fly apart unless they have alot more matter in them that we can not see (but can measure due to its gravitational influence). So, 90% of the matter in galaxies is dark matter. (2) There is a lot of hot X-ray emitting gas in rich cluster of galaxies. The gas should evaporate from the clusters unless there is more mass in the cluster than we see just by adding up the mass of the galaxies. About 90% of the mass in galaxy clusters is dark matter!
We have no idea what dark matter is, but we do know that dark matter contributes about 26% of the density of the universe. So, ordinary matter plus dark matter comprises 4% + 26% = 30% of the critical value. Again, we know the density is the critical value, so where is the remaining 70% of the required density? This was an confusing issue until we realized there is an additional energy density in the universe. We call it dark energy.
The dark energy was discovered from an experiment that should that the expansion of the universe started to speed up with time (and not continue to slow) some 6 billion years ago. This speeding up of the expansion is called "acceleration". We know the universe is accelerating for for two reasons: (1) supernovae in the past appear to be dimmer than they would be in a decelerating universe. This means that they are farther away then would be expected... thus the universal expansion carried them away more rapidly than expected. Thus, we have had a period of accelerated expansion. (2) The change in the amount of X-ray gas in galaxy clusters show that this acceleration had to start about 6 billion years ago.
Ergo, the dark energy, which we do not know what it is, must comprise the remaining 70% of the critical density.
In summary: (new school of thought)
The universe is flat, thus, the density equals the critical value, but this is the sum of three components:
component 1 = ordinary matter = 4% component 2 = dark matter = 26% component 3 = dark energy = 70%which adds up to 100% of the critical density.
However, the expansion of the universe is no longer slowing down, it is speeding up. Thus, at this stage of our understanding, we belive the fate of the universe is that it will expand forever and may actually "rip apart".