Expansion of the Universe and Black Holes

Recently published research has demonstrated that the expansion of the universe and black holes may be linked.

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The universe is a weird place and the latest from cosmology is that the expansion of the universe and black holes may be linked just made it a whole lot weirder thanks to research led by Duncan Farrah from the University of Hawai’i. Back in 2011 Brian Schmidt, Saul Perlmutter and Adam Riess received the Nobel prize for the work they had done in determining that the universe was expanding at an accelerating rate. This was a big deal and painted a picture of a future universe that would just get bigger and bigger and eventually everything would recede from view.

What was causing this accelerating expansion? This was the big question facing cosmologists so a useful placeholder of dark energy was given the role of counteracting gravity and driving the universe to expand. Dark energy is a bit tricky to get your head around but basically it’s the energy of nothing, often called vacuum energy.

Matter, radiation and nothing

The universe is essentially made up of matter, radiation and nothing. Matter is made up of matter we can see, stuff made of protons and neutrons, and dark matter. We can’t see dark matter but it has gravity and we can see the effects of that gravity, so we are either wrong about gravity or there is matter we can’t detect. Radiation is mostly photons and neutrinos and this stuff ran the universe for about the first 50,000 years. Matter took over at that point until about 4 billion years ago, then things got a bit weird. For the last 4 billion years it appears that nothing is now in charge.

Nothing might be something

It turns out that nothing might actually be something, or at least doing something. Einstein developed General Relativity back in 1915 and one of the key components of his field equations was the cosmological constant. This basically assigned mass/energy to empty space. As the universe expanded the energy of empty space increased but the expansion pressure it exerted remained fairly constant. Then about 4 billion years ago the amount of matter and radiation as a proportion had decreased so much due to the expansion of the universe that the energy of nothing started to dominate and drive the expansion of the universe faster and faster.

Expansion of the universe and black holes must have a picture of a black hole - so here it is.
AI generated picture of a black hole (credit: DALL-E2)

Black holes made of nothing?

Models for black holes are generally based around a whole lot of mass squashed into a single point called singularity, but these are not the only models. Way back in 1966 a Soviet scientist Erast Gliner worked out that Einstein’s field equations allowed for a black hole to be made of this weird energy of empty space, vacuum energy. The problem was that he determined a black hole made of squashed matter or vacuum energy would look exactly the same as each other.

Expansion of the universe and cosmological coupling

Duncan Farrah of the University of Hawai’i and his collaborators worked out that the coupling of vacuum energy black holes to the expanding universe could be observed in recently published research. This coupling is called cosmological coupling and if the masses of enough black holes were measured across a time scale of a few billion years and compared to more recent ones then scientists could get an idea if cosmological coupling was real or not. This is what Farrah and his team found, in that very old super massive black holes were much smaller than their more recent counterparts. This could mean that they swallowed more material or merged but Farrah’s team picked elliptical galaxies where there’s not much gas left to consume by hungry black holes and determined that mergers would not make much difference.

Old super massive black holes were smaller

So if the old black holes were smaller than they should be then how did they get bigger? This is where cosmological coupling can explain it. As the universe got bigger the super massive black holes got bigger so maybe these are the ones made from vacuum energy that Gliner theorised. So to check Farrah and his team modelled the amount of vacuum energy provided by these super massive black holes and found that it was in agreement with measurements of the cosmological constant done by the Planck satellite and published in 2018 showing that the expansion of the universe and black holes may be linked.

What makes modern science so amazing is that researchers come up with an explanation for what they observe and then they put it out there for other scientists to pull apart. If it stands up to that scrutiny then we might just have an explanation for the elusive dark energy that is making our universe a whole lot bigger and quicker.

If you want to see what kind of mess a super massive black hole makes then come on a Star Safari when we can see 3C-273.

AI generated picture of a super massive black hole