## Friday, September 10, 2021

### How to measure distances between galaxies in a constantly expanding universe?

10:29 PM | , ,

The universe is constantly expanding, and this has observable consequences.  One is something called redshift.  As galaxies move away from us, the wavelength of light emitted by them stretches, bringing that glow to the red side of the electromagnetic spectrum.  Astronomers can measure how far the wave has shifted red and thus calculate the distance to galaxies.

If you search the distance of some very distant objects, you might find the value of redshift, because it is important in determining whether the body is moving away, and how far it has already moved.  If the object is very old, the tendency is to find a very high redshift in its light.  However, redshift cannot be analyzed in isolation — there are other measures that need to be considered.

If scientists want to correctly map large-scale portions of the universe—large galaxy clusters, or even larger structures like the Hercules-Corona Borealis Great Wall, nearly 10 billion light-years long—they will need to consider small-scale motions. .  There's a lot going on in the universe, so it's necessary to understand the whole picture so you don't get a distorted picture.

Of course, one of the principles of cosmology is for the universe to be isotropic and homogeneous, that is, the same in all directions and the same in all locations.  If we delimit large-scale regions of the cosmos by drawing lines that separate space into several squares a few billion light years each, we would see that all these regions would be identical.  On average, the accuracy would be to about 99.99%, barring only small deviations in the temperature, density and general movement of each object.

The 3D reconstruction of 120,000 galaxies and their clustering properties (Image: Reproduction/Jeremy Tinker/SDSS-III)

This may lead to the assumption that the expansion of the universe is the only factor that can interfere with the real distance of an object on a smaller scale.  Take, for example, the Virgo cluster, the closest to the Milky Way, located 50 million light-years away.  Based on the speed of expansion of the universe, we would be tempted to assume that all the galaxies in the cluster move away from us at approximately 1000 km/s.  Well, it could be that, on average, it could be that the cluster as a structure is moving away at this rate, but there is a catch to this deduction.

When we look at the galaxies that make up the Virgo cluster, we find different relative velocities.  Some are speeding above 2,000 km/s, twice the expected redshift, while others actually have a blueshift—that is, they are approaching us.  This is very strange, but it makes sense if we remember that the universe's isotropy doesn't work so well on smaller scales.

One of the culprits of this paradigm is gravitation.  Things in the universe have been moving since the Big Bang, and they all keep angular motion, that is, they tend to keep moving in the same direction.  But gravitational interactions cause objects to collide with one another, resulting in mergers that create larger objects, or in complex systems of bodies orbiting the same gravitational center—like galaxies.

These movements are also true on even larger scales.  For example, within clusters, galaxies orbit around a common gravitational center, all influenced by dark matter.  Therefore, from our point of view, some galaxies are approaching the Milky Way while orbiting, even though their respective clusters are moving away from our Local Cluster.

For simplicity, we can compare this to a carousel at an amusement park.  While your child is on top of a toy horse, the toy horse system is rotating around a central point.  Meanwhile, you come back from the hot dog stand with a snack in each of your hands, waiting for lap time on the carousel to run out.

As you get closer, the horse your child is riding on approaches you, but at one point it appears to turn left and then move away from you.  However, the carousel itself is stopped and you keep walking towards it.  This is more or less what happens with the individual distances of galaxies in the universe.  We know that their clusters are moving away, but some galaxies, as they move around there, may move closer to us.

That's why if astronomers just consider redshift to measure distances and map the universe, they get an effect called the Finger of God.  The name is suggestive and explains the phenomenon: when looking at large-scale maps of the distribution of galaxies around our position in space, the figure appears to form finger-like structures pointing towards us.  In every direction in the sky that you look at, clusters of galaxies point to Earth.

This can cause serious confusion, as creationists try to use this as an argument for an absolute geocentric universe.  But the FOG effect is just the result of the dynamics of galaxies within their clusters.  That's why astronomers need to be very careful when measuring distances.  While redshift is a very accurate measure and does reveal distance on a large scale, you have to actually measure the distance of an individual galaxy so you don't get a FOG when drawing a map.

Measuring these “real” distances turns out to be a much harder job for researchers, but of course, it's a worthwhile effort.  The reward may be new steps towards a better understanding of what the cosmic web really looks like, as well as ever more accurate maps that allow us to actually know the great galactic clusters throughout the universe.

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