Our Universe Will Probably Not Rip Apart, Phew! (Patreon)

[This is a transcript with links to references.]

The Dark Energy Survey has surveyed dark energy and found that our universe is unlikely to rip into pieces. I think that’s good news. Let’s have a look.

Dark Energy is the name that astrophysicists have given to a hypothetical ingredient of the universe that makes the expansion of the universe faster. So it’s not that dark energy causes the expansion itself, it makes it faster. Dark energy should not be confused with dark matter, which *does contribute to the expansion of the universe but doesn’t make it faster.

Dark energy isn’t like anything we know. I’m not sure exactly why the stuff is even called dark energy, seeing that it’s transparent rather than dark and isn’t even energy, but well, it’s certainly a catchy name. Dark energy is what determines the ultimate fate of our universe, how it will end. Will the expansion get faster and faster? Will it slow down? Is maybe even a recollapse and new beginning possible? It all depends on this mysterious stuff, dark energy.

In astrophysics the name “dark energy” isn’t used for just one thing. It’s a term that encompasses all kinds of different things which could cause this accelerated expansion. One of these types of dark energy is the cosmological constant, usually denoted with a capital lambda. So the cosmological constant relates to dark energy like a banana relates to fruits. It’s one of many.

The cosmological constant is, would you have guessed it, constant, and it’s the simplest type of dark energy. But like there are other fruits besides bananas there are other types of dark energy besides the cosmological constant.

Astrophysicists characterize all stuff that fills the universe by the ratio of its pressure over its energy density. This gives a quantity that’s abbreviated with “w”. Particles that move around fairly slowly like you and I, for example, have zero pressure. That’s astrophysics for you. If you look at your calendar tomorrow, I think you should keep in mind that you have zero pressure.

Dark energy in general has the odd property that while it has a positive energy density, it has negative pressure. For the cosmological constant in particular, the pressure is the same as the energy density, but with the opposite sign, so you have w equals minus 1.

And this brings me to the new data from the dark energy survey. They measured this w by looking at light from distant supernovae. The survey was conducted with a camera mounted on the Victor Blanco telescope in Chile that collects data in 5 different wavelength ranges. It closely monitored about one eights of the total sky for a total of more than 500 nights in the years from 2013 to 2019. In the past few years, the collaboration have analysed the data and are now publishing the results.

In the new paper they report that they found 1635 new supernovae of type Ia. These supernovae have a well-understood light emission that follows a common pattern. They’re what astronomers call “standard candles”.

Astrophysicists believe these supernovae happen when a white dwarfs eats up a nearby star, gets too much, and explodes. Who doesn’t know the problem.If one measures both their brightness of these supernovae and the frequency of their light one can tell how fast supernovae at a certain distance are receding from us. This way, one can use them to infer whether the expansion of the universe is getting faster or not. So, this tells us something about dark energy. It’s the same idea that the 2011 Nobel Prize in physics was awarded for, that was the first evidence for dark energy.

In the new data analysis they now found that the best-fit value for w is minus 0 point 8. And the 95 percent confident band is within plus 0 point 14 and minus 0 point 16 of the best fit value.

Now remember that the simplest type of dark energy is the cosmological constant which has w equals minus 1. So the best fit is barely just outside the 95 percent confidence band. It’s a very mild tension. Kinda interesting, but too insubstantial to think much about.  

More interesting is that it tilts the balance against w being smaller than even minus 1. This matters because the value of w determines the ultimate fate of the universe. With w at minus 1 or larger, the universe will expand faster and faster and get darker and darker and colder and colder in a rather boring way that’s sometimes called the Big Freeze. It might also recollapse, which is called the “Big Crunch”.

But if w was smaller, so more negative than minus one it would be much more dramatic. It’s called the “big rip”. If that happened, eventually everything in the universe would move apart from each other at almost the speed of light, and this means that no forces could hold anything together, not even elementary particles.

The new result from the dark energy survey therefore means that a big rip is unlikely to happen, luckily. I find it kind of hard to understand what a big rip would even mean, but I’m pretty sure I don’t want to be around when it happens. What do you think, Albert?