What's Bigger Than A Universe

When nosotros expect out into the Universe, the stuff we can run into must be close enough for light to take reached usa since the Universe began. The universe is about 14 billion years old, so at outset glance it'due south easy to think that we cannot see things more than 14 billion light years away.

That'due south not quite right, still. Because the Universe is expanding, the most afar visible things are much farther abroad than that. In fact, the photons in the cosmic microwave background have travelled a cool 45 billion light years to go hither. That makes the visible universe some xc billion lite years beyond.

That's large just the universe is almost certainly much bigger. The question than many cosmologists have pondered is how much bigger. Today nosotros have an answer thank you to some interesting statistical assay by Mihran Vardanyan at the University of Oxford and a couple of buddies.

Obviously, nosotros can't directly measure out the size of the universe but cosmologists have diverse models that suggest how big it ought to exist. For case, one line of thinking is that if the universe expanded at the speed of light during aggrandizement, then it ought to be 10^23 times bigger than the visible universe.

Other estimates depend on a number factors and in particular on the curvature of the Universe: whether information technology is closed, like a sphere, apartment or open. In the latter two cases, the Universe must be infinite.

If you can measure the curvature of the Universe, you can then place limits on how big it must be.

It turns out that in contempo years, astronomers accept diverse ingenious means of measuring the curvature of the Universe. One is to search for a distant object of known size and measure how big information technology looks. If it's bigger than information technology ought to be, the Universe is airtight; if it's the right size, the universe is flat and if it'due south smaller, the Universe is open.

Astronomers know of one type of object that fits the bill: waves in the early universe that became frozen in the cosmic microwave background. They can mensurate the size of these waves, called baryonic audio-visual oscillations, using space observatories such as WMAP.

There are also other indicators, such equally the luminosity of type 1A supernovas in distant galaxies.

But when cosmologists examine all this information, different models of the Universe give different answers to the question of its curvature and size. Which to choose?

The breakthrough that Vardanyan and pals have fabricated is to find a way to boilerplate the results of all the information in the simplest possible way. The technique they utilise is chosen Bayesian model averaging and it is much more sophisticated than the usual bend fitting that scientists frequently utilise to explain their data.

A useful analogy is with early models of the Solar System. With the Earth at the middle of the Solar System, information technology gradually became harder and harder to fit the observational data to this model. Just astronomers found ways to do information technology by introducing ever more complex systems, the wheels-within-wheels model of the solar system.

We know now that this arroyo was entirely incorrect. One worry for cosmologists is that a similar process is going on now with models of the Universe.

Bayesian model averaging automatically guards against this. Instead of asking how well the model fits the data, its asks a unlike question: given the data, how probable is the model to be correct. This approach is automatically biased against circuitous models–it's a kind of statistical Occam's razor.

In applying it to various cosmological models of the universe, Vardanyan and co are able to identify important constraints on the curvature and size of the Universe. In fact, it turns out that their constraints are much stricter than is possible with other approaches.

They say that the curvature of the Universe is tightly constrained around 0. In other words, the nigh probable model is that the Universe is apartment. A flat Universe would besides be infinite and their calculations are consequent with this as well. These prove that the Universe is at least 250 times bigger than the Hubble volume. (The Hubble volume is similar to the size of the observable universe.)

That'due south large, but actually more tightly constrained than many other models.

And the fact that information technology comes from such an elegant statistical method means this work is likely to have broad appeal. If so, information technology may well end up being used to fine tune and constraint other areas of cosmology too.

Ref: arxiv.org/abs/1101.5476: Applications Of Bayesian Model Averaging To The Curvature And Size Of The Universe