Invisible matter...

When we look at a group of galaxies like the one in the left (picture credits: Hubble Space Telescope), we are in fact only seeing about 10% of the total mass of that group. This means that, if we count all the mass in the stars and gas of those galaxies, and then compare it with the mass that we measure by other means, we come up short - very short. This means that there is a lot of matter out there, inside and around galaxies and groups of galaxies, that is totally invisible to our telescopes!

One of the best ways to see that there is something missing is to look at the effect of gravitational lensing. When light comes near a very massive object (like a cluster of galaxies), it is pulled towards that object, and the path of the light ray gets deflected. The effect is very much like a lens, which distorts the original object. The more mass there is, the stronger the lensing.

The figures below show the “Pandora” galaxy cluster (Dupke et al. 2011) seen in two different ways: the left picture shows the distribution of light in stars, and the picture on the right shows both gas (in red), which was measured in X-rays, and the mass, which was reconstructed from measurements of the gravitational lensing on galaxies in the background (which appear blue in the picture).

The galaxy cluster “Pandora” (Abell 2744)

The galaxy cluster “Pandora” (Abell 2744)

When we measure the amount of matter in atoms (that is, in stars, planets, gas etc.) in galaxies and clusters of galaxies, we find that it is always approximately five times smaller than the total mass.

... and invisible energy!

In the past 15 years, astronomers have come to realize that the Universe is expanding at an accelerated rate. This was a big surprise: the gravitational pull of the mass of all the galaxies in the Universe should be slowing it down, not speeding it up!

Energy representation

This accelerated expansion only started a few billion years ago, when the Universe was about half its present age. This means that, at some point, matter ceased to be the dominant form of matter/energy, and something else replaced it. This “something else” we call dark energy, and it has the peculiar property that it pushes space itself apart, causing the Universe to accelerate.

Observations of type-Ia supernovas show that the Universe is in fact accelerating (Credit: A. Riess)

Although there are now several pieces of evidence pointing to the amount and strength of dark energy, the first strong hints came from observations of type-Ia supernovas, in 1997 (see figure left).

Dark energy is such an elusive form of matter/energy that scientists cannot rely too much on a single piece of evidence, so we now combine supernovas with many other probes. Presently all clues point to a form of dark energy known as a “Cosmological Constant”, which was in fact conceived by Albert Einstein when he invented General Relativity.

Most of our Universe is invisible!

Presently, we think that the Universe is made of only 4% in atoms (mainly in stars and gas), about 22% in dark matter (which is concentrated around galaxies and in galaxy clusters), and about 74% in dark energy - see the figure to the right. Which means that more than 95% of it remains hidden from us!

Despite the best efforts of astronomers, these numbers still have large uncertainties. Moreover, we are still far away from knowing what are the properties of these mysterious substances. The two problems of figuring out what are the natures of dark matter and dark energy, are among the hardest and most puzzling scientific questions of our time.

Composition of the Universe

As far as we know, dark matter and dark energy are completely invisible (that is, they do not interact at all with atoms). However, these dark components still leave important signs in the visible structures of the Universe (stars, galaxies and so on).

With J-PAS we will look for evidences of dark matter and dark energy in many different places, among them:

• We will observe thousands of type-Ia supernovae, and measure the distances to them without the need of further observations with other telescopes;
• We will measure the effect of gravitational lensing over a huge area of the sky, for millions of objects;
• We will measure in detail the distribution of galaxies in the Universe, both in the angular and in the radial direction;
• We will make the largest inventory of the number of groups and clusters of galaxies; and
• We will measure with unprecedented accuracy the signature of the baryonic acoustic oscillations (BAOs) in the distribution of matter in the Universe.