“I am undecided whether or not the Milky Way is but one of countless others all of which form an entire system. Perhaps the light from these infinitely distant galaxies is so faint that we cannot see them.” -Johann Heinrich Lambert
One of the greatest discoveries of the 20th Century was that many of the great, faint, extended nebulae in the night sky were not merely objects within our own galaxy, like the stars. Rather, these objects were many millions of light-years distant, and were entire galaxies unto themselves.
What was even more surprising is that pretty much all of these galaxies were moving away from us incredibly quickly! Not just hundreds, but thousands or tens-of-thousands of kilometers per second, as evidenced by the redshift of their spectral lines.
And what was maybe most remarkable is that when we graphed the distance to each individual galaxy against its apparent recessional velocity, we found a remarkable relationship.
It was almost immediately realized that this apparent recessional velocity — inferred because of the observed redshift of light — was very likely not due to galaxies speeding away through space, but rather due to space itself expanding.
How does this work? Every element in the Universe — hydrogen, helium, carbon oxygen, neon, etc. — has its own characteristic absorption and emission lines, characterized by the electron transitions within the atom itself. They are fixed quantities for all such atoms in the Universe, and it means that when you see this characteristic series of lines at a given wavelength, you can infer what elements are present.
If this object absorbing these spectral lines is moving towards you, those lines get shifted towards the blue, and if it moves away, they get shifted towards the red, with faster speeds resulting in a more severe shift.
But remember, light itself is simply an electromagnetic wave, defined by a wavelength. And it isn’t just the relative motion between a light source and an observer that can change the wavelength of light; a change in spacetime itself can alter that wavelength as well.
And if you imagine space as a rubber sheet, or (perhaps more intuitively) as the surface of a balloon, you can easily visualize what happens if that sheet gets stretched over time. An object that emits light will have its wavelength stretched proportional to how long that light must travel to reach you. The light that travels twice as far travels for twice as long, and hence gets stretched by twice as much.
This natural explanation for Hubble’s Law is a consequence of General Relativity, and leads to our picture of an expanding, cooling Universe that was hotter and denser in the past, but that will be cooler and sparser in the future.
And as we’ve taken better and better data, we’ve found that Hubble’s Law continues remarkably far into the distant Universe! The expansion rate was faster in the past and has been slowing down, consistent with our predictions from the Big Bang model of the Universe. And when we set the Hubble Space Telescope onto the task of measuring what the expansion rate of the Universe is today, we found that it was 72 km/s/Mpc, meaning that for every Megaparsec away from us (about 3 million light years), a typical galaxy speeds away from us at 72 km/s.
But since those results have come out, we’ve seen farther into the distant Universe. And seeing farther is incredibly interesting, because the farther away you look, the longer the light has been traveling, and hence, effectively, the farther back in time you’re seeing.
And you can measure more precisely — and over a longer baseline of time — exactly how the Universe’s expansion rate has been changing. Based on this, because we understand the laws of physics, we can determine the fate of the Unvierse!
And what we’ve found, as you can see below, is that the Universe isn’t set to recollapse, nor is it even set to be “critical,” where the expansion rate continues to drop, and asymptotes to zero. Instead, our best data indicates that we live in an accelerating Universe!
In an accelerating Universe, you can imagine three separate possibilities, once again, for the fate of it all. Perhaps the acceleration will be uniform, perhaps the acceleration will slow and reverse itself, leading to a recollapse after all, and perhaps the acceleration itself will continue to rise and speed up, ripping the fabric of the Universe apart!
Well, you know me, and I don’t like to speculate; I like to look at the data and see what it indicates! And what it shows, very clearly and unambiguously, is that dark energy — and hence the accelerated expansion — is incredibly, perfectly consistent with a constant.
What does this mean for the future of the expanding Universe? What does it mean for the expansion rate? It means that the expansion rate will continue to drop, but not by that much more. Currently at around 72 km/s/Mpc, the rate will eventually drop and asymtote down to about 60 km/s/Mpc, but no lower than that. Even though the expansion rate is still going down, the expansion itself is still accelerating! How is this possible?
Imagine you’ve got a galaxy that’s 10 Mpc away from you, and the Universe is expanding at a constant rate of 60 km/s/Mpc.
- Initially, this galaxy appears to speed away from you at 600 km/s. Over time, it moves farther and farther away from you, and eventually, it will be twice as far — 20 Mpc — as it was initially.
- But once the galaxy is 20 Mpc away, the expansion rate is still 60 km/s/Mpc. So at 20 Mpc away, the light from it is redshifted as though it moved away from you at 1,200 km/s.
- And this continues; when it’s 100 Mpc away, it appears to move away at 6,000 km/s. When it’s 1,000 Mpc away, it appears to speed off at 60,000 km/s. In other words, even though the expansion rate doesn’t increase, the speed of any individual galaxy increases over time!
- By time a galaxy gets more than 5,000 Mpc away from you, you’ll probably be worried. Because those galaxies will appear to speed off at speeds exceeding 300,000 km/s, which is the speed of light!
What, then, happens at that point?
The galaxies, once they achieve that distance from us, can no longer send us light! The space of the Universe expands too much and too quickly for light to ever reach us from a distant galaxy!
What will this mean for the galaxies in our Universe today? The ones we see peering out into the abyss of deep space?
Yes, they’re beautiful today. But tens of billions of years from now? They’ll all be gone. In fact, if we were born a hundred billion years from now, we would have merged with the Andromeda galaxy and (very likely) the rest of our local group, and thanks to dark energy, there would be no other galaxies or clusters visible anywhere in the Universe!
We are fortunate to live now, as we do, or we may never have discovered the rich galactic structure in our visible Universe, because it will all accelerate away!
So the next time you hear anything about a distant galaxy, no matter where or when, make sure you appreciate it all the more, because billions of years from now, no one in our galaxy will ever know that it exists.