“People hear the happy story, but the truth is they could all disappear in the blink of an eye. The threats just keep coming.” –Todd Steiner
It’s taken generations of scientists, examining the night sky for millennia, to comprehend the full size and scope of what’s out there in the Universe.
Out beyond the planets and stars, outside of the Milky Way itself, is a great cosmic abyss filled with still-uncounted galaxies that stretch for billions of light years across the Universe.
More precisely, there are over 100 billion galaxies in our observable Universe, which is the Universe within about 46.5 billion light years of our present location. The deepest observation we’ve ever made — the Hubble eXtreme Deep Field (XDF) — has recently upped the number of galaxies to closer to 200 billion as a lower limit, with possibly many more than that still lurking undiscovered.
But you might know that the Big Bang, or the birth of all the matter and energy in our Universe, occurred just 13.7 billion years ago, and yet I just (correctly) told you that the observable Universe goes as distant as 46.5 billion light-years from us!
The reason for this is, as light from distant galaxies travels to us, the spacetime between the galaxies continues to expand, something it’s been doing ever since the Big Bang.
With our current understanding of what makes up the Universe — including normal matter, radiation, dark matter, and dark energy — we know exactly how the expansion has worked up until this point, and how it will continue to work in the future. The light that has traveled for five billion years to our eyes shows us an object that’s more like 6.1 billion light years away now, and light that’s just reaching us after a ten-billion-year journey is actually over 16 billion light years away by now.
Because of the way expansion will continue to work into the future, all the objects that are expanding away from us now will expand away increasingly faster in the future, and will eventually disappear from the part of the Universe we can reach.
The Virgo cluster of galaxies — the closest large cluster of galaxies to us — is just 50 million light years away from us, and if we got into a rocket ship that could travel at nearly the speed of light, we’d be able to reach any galaxy within it that we chose in well under 100 million years.
You might think this is true for any galaxy: that if you could travel arbitrarily close to the speed of light, you’d be able to reach any galaxy in the Universe. This would be true if we lived in a Universe without dark energy, but in our Universe, you’ve got your limits, and they only get worse over time.
Because of the expanding Universe, the farther away a galaxy is, the faster it appears to be moving away from us, or the more redshifted its light is. This image — from a section of the Hubble Ultra Deep Field — shows the highest redshift galaxies ever discovered. And yet, you don’t need to be this extreme to already be out of reach: any galaxy with a redshift of 1.8 or more, or that’s more than about 16 billion light years distant right now, is already beyond our reach.
Not even if we had a relativistic spaceship, not even if we could travel at the speed of light ourselves could we reach it. Our first radio broadcast will never be received by those galaxies, and in fact nothing we do from now on can ever affect them. They’re already gone.
In fact, the part that is presently within our reach encapsulates just 4% of the volume of the presently observable Universe!
And it’s only going to get worse over time; as the Universe’s clock ticks by, the objects closer to that edge continue to expand away, receding progressively faster from us. While we might be able to reach the Virgo cluster now, in 100 billion years, it too will be gone. In 100 billion years, I should clarify, this will be all that’s left.
The local group. Ourselves, Andromeda, and our mutual satellite galaxies. That’s it. If we make it to Virgo and are there instead, we’ll have the 1,000-or-so galaxies there comprising the entirety of our visible Universe, with our local group part of the unobservable set of galaxies that’s redded-out by that point.
So think about that the next time you look up.
All the island Universes out there, all the galaxies beyond our own, if we think about the far future, will eventually find themselves alone in the Universe, other than the very few galaxies also in their group or cluster. Each galaxy that we see, with very few exceptions, is only ours to view for a finite amount of time, a blink-of-an-eye in a Universe that promises to be eternal into the future.
And yet, today, here it all is.
All we need to do is look.