(11 am. – promoted by ek hornbeck)
Some say that in the beginning, God created the heavens and the earth. But what do we know about our early universe and how we got here? How do we know that our ideas about the early universe are right? What is dark matter and dark energy and why do we think it exists in the first place?
All of the matter in the universe expanded from a single point. It doesn’t matter much what that means, though. To beg the question is to ask what happened before time began. And because of events that happened during the the inflationary epoch, we can no longer see all of the details of how the universe looked at the beginning of time.
But we won’t ask those questions today. Here we will talk about the current state of cosmology given by The Wilkinson Microwave Anisotropy Probe — the reigning Cosmic Microwave Background Radiation [CMBR] experiment that gives us our best data from the early universe. Within a year, though, we expect a new and improved data set from The Planck Satellite.
Why do we look at the CMBR? What is it? The Cosmic Microwave Background Radiation is the light left over from the big bang — it is the oldest light in the universe. Early on, the universe contained a hot plasma that was opaque to light, but when everything cooled to the point where atoms began to form (an event that is misnamed recombination), the universe became transparent to light. As the plasma cleared up, the light suffered its last scattering, and has traveled on unmolested ever since.
By studying the light left over from the big bang, we can get a picture of what the universe looked like at the time of last scattering, right after the universe cooled enough that the plasma cleared.
The CMBR gives us information about the universe when it was 380,000 years old. What’s so special about that? The physics between the end of the inflationary epoch and the time of last scatter is pretty simple, so we can use the CMBR data to trace the events in our universe back until 10-32 seconds after the big bang. The temperature fluctuations in the light can tell us about the density profile of the early universe which can tell us something about how galaxies formed. We know quite a lot about the content and structure of the universe to a large scale because we can measure this light.
The oval shaped diagram is a map of the temperature fluctuations in the CMBR sky — that is to say if you could see the CMBR temperature fluctuation with your eyes, that is what the sky would look like to you. The anisotropy tells us where the dense points were in the early universe that caused gravitational collapses that clustered into galaxies.
So what do we know?
The universe is 13.73 billion years old, plus or minus 120 million years. We know the age of the universe accurate to within the age of the dinosaurs.
The universe is inflationary. That is to say, of all the self-consistent cosmological theories we have, the ones that include inflation get the CMBR predictions right. We cannot say for sure that inflation is right, but we can rule out all of the theories that get the CMBR wrong — and the inflationary models are the ones left standing.
The universe is flat and highly uniform. WMAP nailed down the curvature of the universe to within 1% of Euclidean flat. That doesn’t mean the universe is shaped like a latke — it means that Euclidean geometry is pretty good on a large scale. Also, ordinary atoms make up 4.6% of the universe.
The universe is dominated by dark energy. Dark energy makes up 72.1% of the universe, to within 1.5%. This means that universal expansion is speeding up — we are undergoing a gentle inflation, so to speak. The universe will not collapse back down into a point as some theories predict.
The pie charts come from fitting the CMBR to cosmological models, and shows the composition of the universe at the time of last scatter as well as today. This is where dark matter and dark energy come from — we see their effects in our measurements. Neutrinos and photons lose energy as the universe expands, so their energy density decreases. Atoms and dark matter become less dense over time as the universe gets bigger. The dark energy density doesn’t appear to decrease very much. Although dark energy didn’t contribute much to the universe when it was young, as the universe aged, it became dominated by dark energy — which accelerates the universe’s expansion. That acceleration is depicted in the timeline diagram above where you can see the bell curving outward as the diagram approaches contemporary time.
We look at the light left over from the big bang in combination with standard astronomic measurements, and we can determine quite a lot about the large scale structure of the universe. Nature was kind to us in that the CMBR light is so promordial that it gives us a picture of what the very early universe looks like. WMAP did a fantastic job in mapping the anisotropy (it superceded COBE), and the Planck satellite started taking data recently — we can expect improved accuracy above the measurements here quite soon.
You can find all of the images in this essay and more at the WMAP website.