—A blogpost about a recent paper by Beatrice Bonga (author of this post), Brajesh Gupt and Nelson Yokomizo—
Have you ever wondered what the shape of our universe is? It turns out that you only need three categories to classify all the possibilities for the shape of our universe: closed, flat or open. The closed category contains all shapes that look like a 3-dimensional sphere or any deformation of it. To visualize this better, let me give you some examples in two dimensions: the surface of a potato and the earth are both deformations of a 2-dimensional sphere. The flat category is like a 3-dimensional plane, with a sheet of paper (whether it is crumbled or not) being an easy to visualize example in two dimensions. The open category contains all shapes that look saddle shaped (or any deformation of it of course).
Is there a way to tell which category our universe belongs to? Observations from cosmology are so far all consistent with a flat universe, which also happens to be the easiest to visualize and do calculations with. This is typically the reason why most data is analyzed using the assumption that that our universe is flat. However, data is becoming increasingly more precise. So is there a chance that our universe is curved after all? We would be like the people of ancient Greece who were able to determine that the surface of the Earth is curved even though it looked flat from their perspective.
This question has been studied by numerous physicists. One of the most amazing data available in cosmology is the Cosmic Microwave Background (CMB). The CMB is radiation emitted when our universe was ~380,000 years old and we are able to observe this radiation now with incredible precision. You could think of it as the baby picture of our universe because our universe is now close to 14 billion years old. To be precise, if you compare the age of our universe with a 100 year old person, the CMB is a picture of a one-day old baby. By analyzing this baby picture carefully, we don’t just learn things about the universe when it was 380,000 years old but also about the years before. During one of those earlier years, the universe underwent a phase of inflation (for more information about inflation, see Anne-Sylvie’s blog post). This phase is important to understand our approach to the question: is it possible that our universe is not flat, but closed?
So how does one usually study the shape of our universe? Typically, when studying the CMB one calculates how the data should look at the end of inflation in their favorite inflationary model and then apply Einstein’s and Boltzmann equations to evolve this data to today. This data is then compared to the baby picture we observe today and the better the match between the evolved data and the actual observations of the CMB, the better the calculated form of our data at the end of inflation was. Scientists so far have looked at the effect of a closed universe on the evolution from the end of inflation to today, but they have not calculated how a closed model changes the data at the end of inflation. This is what we did. We then evolved it with the known evolution equations and compared it to what we observe today.
What did we find? The calculated data at the end of inflation looks different, however, the differences are small and the data remains consistent with a flat model. The differences between the flat and the closed model appear at large scales, for which the closed model does moderately better than the flat model, but at these scales the observational error margins are also largest. Thus, the difference is statistically not very significant.
If you want to know more, you can find the pre-print of our article here. You can also always shoot me an email if you have more questions.