Ask Ethan No. 69: Does the Universe Run Away from Us?

As dark energy dominates, and distant galaxies move away from us with acceleration, what do we lose, and what does this mean for us?

What kind of feeling is it when you leave people, and they become less and less until their specks of dust dissipate before your eyes? - This world is too huge, rising above us with its vault, and this is goodbye. But we are leaning forward towards a new frenzy under heaven.
- Jack Kerouac, On the Road

With age, my goodbyes are getting better and better, but most of us are still not ready for the great truth of cosmic farewell awaiting us. This week I received excellent questions and suggestions, and the honor of receiving an answer goes to Joaquin Bogado, who wants to know about the galaxies disappearing from our field of vision:

In the blog entry “Disappearing Universe” you prompted me to think that many seconds disappear from our Universe every second valuable information. My questions are:
1) how does this affect the Big Bang theory and the age of the universe?
2) Is it possible to find out which part of the universe has already disappeared?

Let's start with the meaning of the disappearance of things, and we will do this by returning to the idea of ​​the Big Bang.



Simply put, the Big Bang creates a hot, dense and expanding Universe in which the fabric of space-time expands. All matter and radiation are diluted, their density decreases, and they fly apart further as the volume of space expands. At the same time, matter and radiation have a strong gravitational effect, trying to collect the universe back.

This is the great cosmic struggle between expansion and gravity. For billions of years, the observer could not be sure which one would win.



Will gravity win, because of which the Universe will reach its maximum size, reverse the expansion and recollapse in Great compression?

Will expansion win, because of which the Universe will expand forever, never ceasing, pushing everything to arbitrarily large distances from each other, and ending with the Big Freeze?

Or we will live in a case located just on the border of the first two, where only one additional atom would lead to a recollapse, where the expansion rate asymptotically tends to zero, but never reverses: the critical Universe.



And although these outcomes are very different, they have one thing in common. Take a look at today's Universe, at any of the galaxies. All that you can see - on the very brink of visibility - is a galaxy whose light reaches us only now, having made its way through the universe.

A lonely photon, having spent billions of years traveling through an expanding space dividing us, eventually gets into our eyes. After all this time spent in a swim against the current, against the expanding Universe, he caught up with us.



Over time, in the first 7.8 billion years of the universe, light from an increasing number of galaxies has caught up with us.

Why?

Because the expansion of the Universe was slowing down - that is, despite the expansion, and the constant removal of galaxies from us, the speed of their removal was constantly decreasing. As a result, galaxies that were initially invisible to our eyes because of too far a distance became visible to us.



Over time, it became possible to observe an increasingly large part of the universe. If only matter and radiation were present in it, this would go on forever, regardless of the outcome awaiting us. We would have access to more and more, the slowdown would continue, and the question would be only what would happen to the expansion. It would:

• reached zero, and then would turn (Large compression);
• decreased, but remained positive, and everything would have scattered even further (Big freeze)
• asymptotically tended to zero, but did not reach it (Critical case)

But with the decrease in the energy density that occurs as the Universe expands, something surprising has become evident: space has its inherent energy, a new type of it: dark energy.



Only when the density of matter and radiation fell dramatically - which took billions of years - did this dark energy become noticeable, and it took 7.8 billion years from the Big Bang to change the history of space.

It was at that moment when the density of dark energy became large enough to reach a third of all energy in the Universe, instead of slowing down, galaxies began to fly away from us with acceleration. This means that their speeds do not decrease, but increase!



The light from the galaxies, from which he has already managed to reach, continues to reach - the acceleration of the Universe will not change this. But we will not see a new light from many galaxies - only that light that they emitted a very long time ago. Think about why.



A distant galaxy emits light in an expanding universe. The space between us and this galaxy continues to expand, but the photon is still flying towards us. Since the galaxy constantly emits light, it will not only reach us now - in the future there will also be light that will reach us!

But think about where this galaxy is today. Think of an expanding, accelerating universe. Think about how big it is today.



This picture is almost relevant: the diameter of the observable part of the Universe is approximately 92 billion light years, and it contains at least hundreds of billions (possibly trillions) of galaxies.

The thing is that any galaxy that is more than 14 billion light-years distant from us no longer emits the light we see: the expansion of the space between this object and us is so fast that the photon emitted today never reaches us! By calculating the volume of the observable Universe contained in a sphere with a radius of 14 billion light years, and comparing it with the contents of a sphere with a diameter of 92 billion light years, you will find that we are associated with only 3% of all galaxies: all the rest are lost forever!



Over time, more and more galaxies and clusters will leave our horizon. This does not mean that we will stop seeing them, it means that we will not be able to get to them. Unless we have an ultra-relativistic spacecraft, unless we can send them anything at the speed of light.

But the light that we emitted billions of years ago can still reach them. Only now, in exactly the same way as in the case of the light coming from them to us:

• its quantity is limited;
• he experiences a very strong redshift;
• time in it is stretched, that is, events are stretched in time;
• its expiration with time is less and less.

To detect these distant redshifted galaxies, we need to “leave the shutter open” longer.



If there were no dark energy, there would be no acceleration and disappearing galaxies. The big bang could have happened in exactly the same way, but our Universe today would have been much smaller, the galaxies would have been closer, we would have seen more of them, they would have not been so shifted into the red spectrum, and the Universe would have expanded not so fast, and each galaxy would slow down. Instead of galaxies disappearing about one in three years, we would have more and more opened!

And although not a single galaxy has completely disappeared from sight, 97% of them have disappeared in the sense that they have become unattainable, and that the light emitted by them today will never reach us. They are still visible, but only because of their old world.



This is how our fading Universe works, and this is what it means when galaxies disappear from our field of vision. Thank you for the wonderful question, and I hope that the explanation has been made clear to you and the rest. Send me your questions and suggestions for the following articles.