Mapping the geography of the Universe

When we look at the sky on a clear night, and we see a bright strip of light across the sky, we are actually looking at the light from billions of stars that make up the Milky Way. The stars of the Milky Way can be so distant, and so small, that we can’t see them individually, so most of them appear to us as blurred smudges. These smudges occupy an elongated stripe in the sky, which means that stars are not evenly distributed, but they concentrate on some regions more than others. This region is our galaxy, the Milky Way, which appears to us as a huge band in the sky because it has the form of a thick disk with a bulge in the middle, and we are looking at this disk from inside it (the Sun is roughly midway from the bulge to the outer limits of the galaxy).

When we look out of the disk of the galaxy, we start to see more clearly objects which are not in the Milky Way, such as other galaxies. And, if we look very carefully and map a large number of these galaxies, we notice that the galaxies themselves are not distributed homogeneously across the Universe: they are more often found close to other galaxies, in groups or clusters of galaxies.

Atoms, stars, galaxies... the Universe!

When we look at vast distances across the Universe, we see hundreds of millions of galaxies. It doesn’t matter how far we look, we always seem to find more galaxies - and, because the speed of light is finite, far away in space means far back in time!

When we look at the distribution of galaxies in the Universe, what we see is a web-like structure, where the fibers of the web are made of hundreds or thousands of galaxies. Gravity is the driving force that pulls galaxies towards each other, creating rich regions with many galaxies, and emptying vast bubbles known as voids, where hardly any galaxy can be found.

But are these galaxies telling us anything? What is their distribution, and why have these galaxies formed in some places rather than others? These are ultimately questions about our own origins. Studying the distribution of galaxies can tell us a lot about how the Universe began, and what it is presently made of.

Large-scale structure and BAOs

The distribution of galaxies in the Universe is approximately described by a power-law: what this means is that the likelihood of finding a pair of galaxies separated by some distance (d) is roughly proportional to that distance to some power, d p . This is a consequence of the fact that the Universe began as a very homogeneous, very smooth fluid, where all places had almost the same density, but then, as gravity pulled heavy stuff together, first stars, then galaxies, then clusters of galaxies started to form. The final result, today, is that galaxies are distributed according to this power-law. One of the main science drivers for S-MAPS is its ability to make accurate measurements of this power-law.

However, some funny business took place in the very early Universe, which has deformed that simple power-law. When the universe was less than half a million years old, atoms (baryons) were so hot that they formed a single fluid together with light (photons). The Universe was also a very opaque place, because all atoms were ionized, so light would scatter very often on the free electrons and ionized nuclei, so photons would effectively become “trapped” in denser regions. What happened, then, was quite amazing: the pressure exerted by the trapped photons would compensate the pull of gravity, and these dense regions would breathe in and out, oscillating like a pendulum. Those oscillations are called baryon acoustic oscillations (BAOs), and the times and length scales associated with BAOs are a unique relic of those ancient times. Observing these BAOs is one of the holy grails of modern Cosmology.

After the Universe cooled down, and ionized nuclei were able to capture the free electrons to make neutral atoms, these oscillations froze, and their mark was imprinted on the initial density of matter in the Universe at that early epoch. Today we can see the remains of the BAOs in the distribution of galaxies.

The role of S-MAPS

The best way to measure BAOs is to map very carefully the types and positions of galaxies in the Universe. Since the BAO features are quite small, we need to map many millions of galaxies over distances of billions of light-years, in order to beat the statistics and reach a good accuracy in that measurement. We also need to reach deep in the Universe, in order to measure the BAO features at different times, and observe how the Universe went from a decelerated phase (dominated by atoms and dark matter) to the present accelerated phase (dominated by dark energy).

S-MAPS and J-PAS will be the ultimate instruments to do this massive, precise 3D mapping of the Universe. These instruments will be able to observe hundreds of millions of galaxies, out to distances of more than 3 Gpc (that is, more than 9 billion light-years). At the end of these surveys, we will not only know much better how our Universe began and evolved in time, but we will also have a near-complete map of our observable Universe.

While professional astronomers will be busy exploring the catalogs and statistical properties of this survey, we will also make sure that the general public can travel inside this 3D map of the Universe, visiting places and times that, until recently, we could only dream of.