Mysteries Of The Universe
BBC Earth|March - April 2021
In the last decade, we’ve taken photos of a black holes, peered into the heart of atoms and looked back at the birth of the Universe. And yet, there are yawning gaps in our understanding of the Universe and the laws that govern it. These are the mysteries that will be troubling physicists and astronomers over the next decade and beyond
Marcus Chown

1 WHY IS THERE SOMETHING RATHER THAN NOTHING?

In the beginning, according to the standard picture of cosmology, was the ‘inflationary vacuum’. It had a super-high energy density and repulsive gravity, causing it to expand. The more of it there was, the greater the repulsion and the faster it expanded. In common with all things ‘quantum’, this vacuum was unpredictable. At random locations, it decayed into the ordinary, everyday vacuum. The tremendous energy of the inflationary vacuum had to go somewhere. And it went into creating matter and heating it to a blisteringly high-temperature – into creating big bangs. Our Universe is merely one such Big Bang bubble in the ever-expanding inflationary vacuum.

Remarkably, this whole process could have started with a piece of inflationary vacuum with a mass equivalent to a bag of sugar. And, conveniently, the laws of physics – specifically, quantum theory – permit such matter to pop into existence from nothing. Of course, the next obvious question now is: where did the laws of physics come from?

In 1918, German mathematician Emmy Noether shed light on this. She found that the great conservation laws are mere consequences of deep symmetries of space and time – things that stay the same if our viewpoint changes. A striking property of such symmetries is that they are also symmetries of the void – of an entirely empty Universe. So maybe the transition from nothing to something was not such a big deal. Maybe it was simply a change from nothing to the ‘structured’ nothing of our galaxy-filled Universe. But why did the change happen? The American physicist Victor Stenger pointed to the fact that, as the temperature drops, the water turns into structured water, or ice, because ice is more stable. Could it be, he speculated, that the Universe went from nothing to ‘structured nothing’ because structured nothing is more stable?

2 WHY IS THERE A MONSTER BLACK HOLE IN THE HEART OF EVERY GALAXY AND DOES IT HAVE ANYTHING TO DO WITH US BEING HERE?

There are about two trillion galaxies in our Universe and, as far as we know, almost every one contains a central supermassive black hole. They range in size from monsters, weighing almost 50 billion times the mass of the Sun, to the 4.3-million-solar-mass tiddler known as Sagittarius A* in the core of our Milky Way (one solar mass = mass of our Sun). But how they got there is one of the great unsolved mysteries of cosmology.

We know that a stellar black hole forms in a supernova explosion in which the core of a star implodes. But nobody knows how a supermassive black hole forms. For most of cosmic history, the centres of galaxies have been where a lot of matter is confined in a small volume. It could be the case that supermassive black holes form in a dense star cluster out of stellar black holes which repeatedly merge with each other. Tentative evidence for this comes from a merger between two black holes revealed by a recent detection of gravitational waves. One hole was too big to be a supernova relic and so may have originated in an earlier merger.

An alternative way to form a supermassive black hole is from the direct shrinkage of a dense cloud of gas. It could be that they form from a combination of cloud collapse and black hole mergers. It is also possible that supermassive black holes formed in the Big Bang. This would provide a novel answer to the cosmic chicken-and-egg question: which came first – galaxies or supermassive black holes? Rather than galaxies forming first and then spawning such monsters, supermassive black holes would form first and provide the seeds about which galaxies of stars formed.

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