The most powerful forces in the universe.

All About Space runs for cover as we explore the objects in the cosmos that pack the biggest punches of all.

The universe is an incredibly violent place, populated by explosions and torrents of radiation, pulled this way and that by powerful fundamental forces, and lit up by active centres of galaxies and massive stars. All these forces are in interplay -supernovas create black holes, while gravity battles dark energy to decide the fate of the universe. Energies far greater than the Sun can produce in 10 billion years are wielded in a matter of seconds, and our knowledge of physics is put to the test by the most extreme and most powerful events in the universe.

THE EXPLOSION THAT CREATED THE UNIVERSE.

The big bang.

Our universe sprang into existence around 13.77 billion years ago; a great event that created everything we know of — from stars and galaxies to planets and Solar Systems. Nothing existed before the Big Bang. While it’s easy to imagine that a great explosion created our universe, this is far from the truth. Currently we understand that, at first, there was nothing and, during and after that moment, time and space came into existence -beginning as an infinitesimally small, infinitely hot and dense object. Just where it came from, is however, something experts are still not sure of.

What we do know is that this point began to expand and is continuing to do so according to the rate at which galaxies are moving away from us. The story of how the cosmos came to be as it is today is a tale of high energies, thick ‘fog’ and sizzling temperatures which gradually calmed, cleared and cooled, creating the first particles and the beginnings of the fundamental forces that surround us. These are the electromagnetic, weak, gravitational and strong forces, the latter being the one that holds nuclei together.

As the universe cooled further it shifted from being radiation dominated to being matter dominated, introducing the hydrogen atoms along with the cosmic microwave background radiation — the thermal radiation that fills every part of the universe — which crackles its presence when radio dishes are turned upon it.

The final transformation saw the emergence of large-scale structures as the earliest stars, quasars, galaxies, clusters of galaxies and superclusters were added to the cosmic mix.

POWERING THE EXPANSION OF THE UNIVERSE.

Dark energy.

We can’t see it, but we know it’s there. The mysterious dark energy, which accounts for roughly 70 per cent of the universe, is the driving force behind why galaxies are moving away from us in an almost eternal expansion, which, according to experts, isn’t showing any signs of slowing down.

Permeating through every corner of space, scientists didn’t even realise it existed until 1997. Two groups of astronomers had been competing against each other to measure the expansion rate of the universe by using the light of supernovas. As the universe expands, the light is stretched and reddened. Because certain types of supernovas — the explosions of merging white dwarf stars — detonate with practically identical energy and luminosity, they believed it would be possible to measure their ‘redshift’ and consequently the expansion of the universe. They expected it to be slowing down — instead it was found that it was actually speeding up!

Nobody knows what dark energy is or even precisely how strong it is. It acts a bit like anti-gravity, pushing the universe apart. On the biggest scales it overcomes all of the other forces in the universe, including gravity, and that could prove to be bad news for the universe. If dark energy was to become too powerful, it could tear the universe apart in a ‘big rip’, starting with galaxy clusters, then galaxies themselves, then stars, planets, us and even our constituent atoms until the fabric of space and time itself is destroyed completely.

At best dark energy will accelerate the expansion of the universe so that every other galaxy is moved so far away from us that we will no longer be able to see them, but astronomers need not panic yet — this is not expected to happen for approximately another 2 trillion years.

GRAVITY.

THE FORCE THAT BINDS THE UNIVERSE TOGETHER.

In Star Wars, there was The Force — the mystical field that binds together all life. In the universe, however, there is another ‘force’ that binds together all matter, and that’s the somewhat mysterious force of gravity. That famous (and probably false) story of an apple falling on Isaac Newton’s head was only the beginning of gravity’s remarkable story.

What makes the planets round? Gravity. What keeps us from floating away? Gravity. What causes temperatures and pressures to grow so high in the core of the Sun that it can ignite nuclear fusion? Gravity. What keeps the planets orbiting the Sun? Gravity. And so on.

So, gravity is a big deal. Newton’s laws of motion and his law of universal gravitation describe how gravity operates in everyday life. However, things can get a little strange when we start to talk about really massive objects, or things that are moving at close to the speed of light. This is where Einstein’s general theory of relativity comes in, describing such concepts as gravitational time dilation, black holes and neutron stars with immense gravity, gravity wells in space-time, and gravitational lenses where massive objects like galaxy clusters are able to bend and magnify the light of more distant objects. And when neutron stars merge, or black holes crash into each other, they unleash a torrent of ‘gravitational waves’ that ripple through spacetime. Nobody has ever detected a gravitational wave (see our space mysteries feature on page 54) , but scientists are always on the lookout and hope to meet with some success in this area in the coming years.

Oddly, for a force that is so important, gravity is quite weak on small scales.

A bar magnet, for example, can overpower gravity, picking bits of metal up for fun. But on much larger scales gravity dominates, holding entire galaxy clusters together. It’s only when it comes face to face with the ever-growing force of dark energy that gravity starts to become unstuck. ultimately, the fate of the universe will be decided by the battle between gravity and dark energy: will dark energy rip the cosmos apart, or will gravity be strong enough in the long run to pull the universe back in a ‘big crunch’? The end of the universe may be decided by one of these theories.

SO BRIGHT THEY CAN BE SEEN FROM THE EDGE OF SPACE.

Quasars.

They might be distant, but packing a punch of high energy and indescribable luminosity are quasars — objects believed to be glowing strongly since their creation in the universe’s early days.

Usually found in the very centres of active galaxies, quasars are among the most powerful objects in the universe; with most throwing out a luminosity equivalent to around 2 trillion Suns, while others emit strongly as sources of radio emission and gamma rays. So what gives them so much power?

In the nuclei of the galaxies they occupy, a supermassive black hole munches on the material from the disc of gas around it. This gas is then fed into the centre of the galaxy with the dazzling quasar light all coming from this million-degree hot disc and the jets of energy it unleashes. The jets form because the disc is a tangle of magnetic fields that become tightly wound as the disc rotates, trapping charged particles within them until they’re fired out at almost the speed of light. It’s only when we look almost head-on at these jets that we see a quasar. Indeed, they are so bright and powerful they can be seen right across the known universe.

THE POWER TO HARNESS A GALAXY.

Supermassive black holes.

The ultimate consequence of gravity is a black hole. Imagine a region of space where gravity has caused a star to collapse at the end of its life to a point so small and dense that its gravity is practically infinite and completely overwhelms everything else. It’s so strong that not even light can escape its grasp — the point of no return is known as the event horizon — explaining where the name black hole came from. And black holes don’t come any more massive than a hefty supermassive black hole. With a mass ranging anywhere from hundreds of thousands to billions of times the mass of the Sun, these exotic high-gravity objects are, more often than not, the centrepiece of the many galaxies that litter our universe. Our own Milky Way even has one, called Sagittarius A*, which is a monster of around 4.3 million times the mass of our Sun, located deep in the middle of our galaxy amid myriad stars and vast clouds of gas and dust. So powerful are these galaxies that they have the strength to switch star formation in a galaxy on and off at will.

Think back to quasars — these are the most extreme form of active supermassive black hole. But less energetic black holes can still produce lower power jets, yet even though they’re lower power, they still dominate the galaxy that they are in. Stars need gas to form, and the gas in galaxies often falls on to them from wandering clouds of intergalactic gas. Yet as clouds fall on to galaxies, and as the galaxies merge with other galaxies, gas gets funnelled towards the black hole, ending up in a disc surrounding it, some of which is then beamed back out into the galaxy by jets, or ‘winds’, of stellar radiation.

These jets and radiation heat the gas that is creating stars, causing it to become too hot for star formation and sometimes even blowing right out of the galaxy itself. This is called feedback, and when it happens it brings star formation in a galaxy to a stuttering halt.

THE ENERGY OF A THOUSAND SUNS

Gamma ray bursts.

Gamma-ray bursts (GRBs) signal the biggest explosions in the universe. They were discovered in 1973, after analysis of data from the Vela satellites, which the USA launched to try to detect Soviet nuclear tests in space. Instead, they found ferocious bursts of gamma rays from outer space. It took almost 25 years to figure out what they were. Scientists call them ‘collapsars’. When the most massive stars of all reach the end of their lives, they can no longer hold back gravity and their core collapses into a black hole. Gas from some of the layers surrounding the core rain down on to it and, just like how the black holes in quasars produce jets, so do the black holes inside the collapsing star. All of this happens in merely a fraction of a second, and the jets blast out through the star’s outer layers at close to the speed of light, as the star explodes.

But this isn’t what creates the gamma rays. The jets are formed from highly entwined magnetic fields, and when charged particles like electrons and protons spiral around magnetic fields like this, they produce gamma rays, and it’s these gamma rays that we see as a burst. As for their power, there’s nothing else like them in the known universe, releasing the equivalent amount of energy that a thousand Sun-like stars will release over their entire lifetimes! If a GRB’s energy could be harnessed on Earth, it would meet the world’s energy demands for billions of years.

THE POWER OF AN EXPLODING HYPERGIANT.

Hypernovas.

As you might imagine, the supernovas that create gamma-ray bursts are no ordinary type of exploding star; instead we call them hypernovas, and they make normal supernovas look like a mere pop in comparison.

Hypernovas can be 20 times more luminous and up to 50 times more energetic than a normal supernova.

It’s not entirely clear why hypernovas are different to normal supernovas, but mass undoubtedly has something to do with it: some stars like Eta Carinae have masses around 100 times that of our Sun, while other stars that explode as supernovas may have only a dozen solar masses. In addition, massive stars that have been deprived of heavy elements -elements heavier than hydrogen or helium — have a tendency to explode as hypernovas.

However, not all hypernovas create gamma-ray bursts. It seems the most extreme examples are completely annihilated without even leaving a black hole behind. These are known as ‘pair instability’ supernovas and happen when electrons and their antiparticles — or positrons — are formed en masse by collisions between energetic gamma rays and atoms inside the dying star.

Not only does this lead to reduced pressure inside the supermassive star, which then prevents the core of the star from fully collapsing to create a black hole, but when matter and antimatter come into contact with each other in this way they create what’s known as a runaway thermonuclear explosion that utterly destroys the star, without leaving a black hole remnant behind.

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