The End of Everything (Astrophysically Speaking)

by Katie Mack

Acknowledging an ultimate end gives us context, meaning, even hope, and allows us, paradoxically, to step back from our petty day-to-day concerns and simultaneously live more fully in the moment.

if we want to learn about the evolution of the universe itself, and the conditions our own Milky Way galaxy grew up in, all we have to do is look at something far away.

If we measure a galaxy’s redshift, we know how quickly it’s receding from us, and we can use the Hubble-Lemaître Law to get its distance. But because light takes time to travel to us, and we know light’s speed, knowing the distance also tells us how long the light has been en route.

Because we don’t know whether it’s really a cosmological constant or not, we generally call any hypothesized phenomenon that could make the universe accelerate in its expansion dark energy.

The reason a cosmological constant dooms the universe is that once it starts, the accelerated expansion never, ever stops.

The present-day observable universe is probably bigger than you think. The “observable” part refers to the region within our particle horizon. We define this as being the farthest we could possibly see, given the limitations of the speed of light and the age of the universe.

Knowing that the universe is about 13.8 billion years old, logic would tell you that the particle horizon must be a sphere of radius 13.8 billion light-years. But that’s assuming a static universe. In actual fact, since the universe has been expanding all that time, something just close enough to send its light to us 13.8 billion years ago is now much farther away—approximately 45 billion light-years. So we can define the observable universe to be a sphere of about 45 billion light-years in radius, centered on us.

While nothing can travel faster than light through space, there’s no rule that limits how quickly things can happen to find themselves farther apart because they are sitting still in a space that’s getting bigger between them.

When the universe gets to this pure de Sitter state, it is a maximum entropy universe. From that point on, there is no way for the universe’s total entropy to increase, which means, in a very real sense, the arrow of time is… gone.

If dark energy is a cosmological constant, its defining feature is that the density of dark energy in any given part of space is constant over time, even as space expands. The expansion rate isn’t constant, just the density of the stuff itself, in any given volume of space.

if w is even infinitesimally lower than -1, dark energy will tear the entire universe apart, and it will do so in a finite, calculable time. I just want to pause for a moment to say that this paper, titled “Phantom Energy: Dark Energy with w < -1 Causes a Cosmic Doomsday,” is one of my absolute favorite papers in physics.

the difference between a Heat Death–fated universe and one headed for a Big Rip might literally be unmeasurable. If dark energy is a cosmological constant, the equation of state parameter w equals -1 exactly, and we get a Heat Death. If w is at all lower than -1, even one part in a billion billions, dark energy is phantom dark energy, capable of tearing the universe apart.

We can see Cepheid variable stars throughout the Milky Way and in nearby galaxies, so we can use parallax for the nearby ones, carefully calibrate the pulsation relationship, and then use the more distant ones to tell us the distances to other galaxies.

we have at least tens of billions of years before even the most extreme version of a sudden Big Crunch reversal could occur, and no Big Rip could be less than a hundred billion years off.

In the very early universe, the Higgs field underwent a transition that separated the electroweak force into electromagnetism and the weak force, and in the process gave some particles (though not the photon or gluon) the ability to interact with the Higgs field itself. The strength of that interaction determines the particle’s mass.

If a vacuum decay event happens at one place in the cosmos, it causes a bubble to expand outward at the speed of light, destroying everything in its path.

The upcoming LSST (Large Synoptic Survey Telescope), recently renamed the Vera C. Rubin Observatory (VRO), is a fantastic example. An 8.4-meter telescope on a high-desert mountain in Chile, VRO will take images of a few million supernovae and 10 BILLION galaxies,