Astronomers today know a tremendous amount about the universe – but it still has many questions and mysteries. Here are 20 of the biggest secrets in space…
Written by Giles Sparrow
1 How big is the universe?
Less than a century ago, most astronomers believed that our Milky Way galaxy, roughly 100,000 light years in diameter, was the entirety of the universe – it was only in the Twenties that Edwin Hubble used rare stars called Cepheid variables to show that the spiral nebulas in the sky were actually independent galaxies millions of light years beyond our own. Since then, the universe has just got bigger and bigger – modern giant telescopes can now see galaxies many billions of light years away.
The main thing that puts a limit on the size of the universe is the limited speed of light, and the fact that it’s only had 13.8 billion years to grow since the Big Bang. Any light from objects more than 13.8 billion ‘light years’ away simply hasn’t had time to reach us yet, and this limits our ‘observable universe’ to a spherical bubble with a radius of 13.8 billion light years, centred on Earth. How far the universe might carry on beyond the observable boundary, though, is still hotly debated.
2 What’s inside a black hole?
A black hole is an object with such high mass and density that its ‘escape velocity’ (the speed an object would have to travel to leave its surface) is faster than the speed of light, so that nothing can escape from it. Although the very nature of black holes meant that they were purely theoretical objects for several decades after their first prediction in 1915, since the Seventies astronomers have identified black holes with increasing certainty from the effect they have on their surroundings.
The outer ‘surface’ of a black hole is called its event horizon, and marks the point at which its escape velocity becomes greater than the speed of light. But the event horizon isn’t the surface of a solid object – most astronomers think that objects heavy enough to form black holes (usually the collapsing cores of burnt-out monster stars) have such powerful gravity that they crush their constituents into subatomic smithereens – tiny particles that are known as quarks. These collapse to form an incredibly tiny but dense point known as a singularity, which bends space and time around it in strange ways.
3 How is there water on Jupiter?
In 1995, the Infrared Space Observatory satellite detected signs of water in Jupiter’s stratosphere, the uppermost layer of its atmosphere. Jupiter’s lower atmosphere appears to be dry, and while models of its internal structure predict large amounts of water beneath the clouds, cold layers above should prevent it from ‘leaking out’. In 2013, the Herschel Space Observatory mapped Jupiter’s water signature and found that it is concentrated over regions that were in the firing line when Comet Shoemaker-Levy 9 hit Jupiter in 1994. While the visible scars from this cosmic crash have long since faded, the comet seems to have left a lasting impression!
4 The origins of gammaray bursts
High-energy gamma ray bursts (GRBs) from space were discovered in the late-Sixties by satellites designed to monitor nuclear tests on Earth. They seem to come from far beyond our Milky Way galaxy, but it was only in 1998 that a GRB was finally linked to the flare of a supernova in a faint distant galaxy for the first time. Today it seems that most bursts come from supernovas, but a substantial minority come from neutron stars colliding and merging to form black holes. However, the mechanism that produces the gamma rays, and focuses them into tight beams that can cross billions of light years of space to Earth, is still not fully understood.
5 Where comets come from
Bright comets are rare but spectacular visitors to Earth’s skies, so it’s little wonder they made an impression on our ancestors. For centuries, people thought they were atmospheric phenomena, but in 1577, Danish astronomer Tycho Brahe showed for the first time that comets lay far beyond the Moon. Then in 1705, Edmond Halley computed the path of the comet that bears his name, predicting that it took around 76 years to orbit the Sun in a highly stretched or elliptical orbit.
Today we recognise that most comets follow elliptical paths. Some, like Halley’s, have relatively short orbital periods and reach their most distant point from the Sun a little way beyond the orbit of Neptune, amid the region known as the Kuiper belt. Other comets have much longer periods and may travel hundreds of times further from the Sun. Occasional rare visitors are not in orbit around the Sun at all – they are rogue interstellar comets that make a single pass through our Solar System.
In 1950, Jan Oort realised that the orbits of long-period comets suggested they were coming from a huge cloud surrounding the Solar System at a distance of 20,000 AU or more. Even though we can’t see this cloud, we now think it’s the origin of all the Solar System’s native comets, and was probably created when the planets ‘kicked out’ comets from closer to the Sun early in the Solar System’s history. Today, encounters with giant planets can pull long-period comets back into shorter-period orbits.
6 What is the aurora borealis?
The beautiful glow of the northern lights is one of the most entrancing spectacles in the night sky – though it’s one that’s rarely visible across most of the UK. Many of the early theories that tried to explain these shimmering patterns of light viewed them as an unusual type of weather, and it was not until 1900 that Norwegian scientist Kristian Birkeland suggested that the auroras were created by particles from the solar wind entering the Earth’s atmosphere.
Unfortunately, Birkeland’s specific theory was wrong, and it was not until the dawn of the space age that astronomers and geophysicists first began to understand the true nature of how the auroras are created. American scientist James Van Allen pioneered the study of Earth’s magnetosphere using instruments carried aboard early NASA satellites to discover the Van Allen radiation belts of fast-moving, high-energy particles, that surround the Earth.
Thanks to the work of Van Allen and others, we now understand that the auroras are created by the interaction of our planet’s magnetic field with that of the Sun itself, carried across the Solar System on the solar wind. This allows some energetic particles to slip down Earth’s magnetic field lines in ‘auroral ovals’ around each pole. About 80 kilometres (50 miles) above the ground, these particles strike atoms of nitrogen and oxygen in the thin upper atmosphere, temporarily boosting their energy. As the atoms return to their normal state, they release excess energy in the form of light with characteristic wavelengths – the most common auroral colours are green or brownish red (from oxygen) and blue or red (from nitrogen).
But even if we understand the mechanism behind the lights themselves, the auroras still have mysteries. For example, there are anecdotal reports of sounds such as claps and crackles accompanying auroras. Researchers were sceptical, but in 2012 a Finnish team succeeded in recording the sounds for the first time. What causes them, however, remains unknown.
7 What are Fermi bubbles?
In 2010, a team using the Fermi Gamma-ray Space Telescope discovered two huge bubbles of high-energy gamma-ray emission, each roughly 25,000 light years in diameter, extending above and below the Milky Way. The bubbles had been hidden from view by an intervening ‘haze’ of gamma rays from nearby space, and when a team working on models to explain the haze developed a way of peering through it for the first time, they found the enormous ‘Fermi bubbles’ that lie beyond.
The well-defined structure of the bubbles and the strength of their emissions suggest they were formed in a single rapid event, perhaps a few million years ago, close to the crowded centre of our Milky Way galaxy. Since then, they’ve ballooned outward into the mostly empty regions of the galactic halo. Despite appearances, they really are thin-walled bubbles of gamma-ray emitting material, rather than filled-in clouds.
The discovery of a gamma-ray-emitting jet linking the bubbles to the centre of our galaxy provided crucial evidence for the event that created them – a burst of activity from the supermassive black hole at the centre of the galaxy. This invisible monster is normally starved of material – but when a stray gas cloud or larger object passed nearby, it seems that it belched out bubbles of stray hot gas as it gobbled down a rare meal.
8 Why does the Sun have cycles?
The solar cycle was discovered in 1843 by German astronomer Samuel Heinrich Schwabe, following years of carefully observing dark sunspots on the surface of the Sun. He found that sunspots start each cycle in small numbers at relatively high latitudes on either side of the Sun’s equator, increase in numbers as they moved towards the equator, and then fade away as they reached the lowest latitudes, before reappearing at high latitudes once again. At first glance, the cycle appears to repeat every 11 years, but measurements of the magnetism of the spots shows that their north-south polarities reverse with each cycle, so the entire sunspot cycle takes 22 years to complete. Since the cycle was discovered, it’s also become clear that it affects many other aspects of solar activity, with solar flares, X-ray emissions and violent ‘coronal mass ejections’ all at their peak around the same time as the sunspot maximum.
The cycle is driven by changes in the solar magnetic field, which is created by electric currents flowing in the upper layers of the Sun’s atmosphere. At the start of each cycle, the magnetic field is neatly aligned from pole to pole, but the Sun’s rotation causes its fast-spinning equator to drag the field around the Sun until it becomes tangled. Tangles and loops in the magnetic field are responsible for the sunspots and other activity. While this basic mechanism is understood, astronomers are still struggling to understand how solar cycles can vary hugely in intensity and sometimes even disappear completely for several decades.
9 Are there more than three dimensions?
We know for certain that there are at least four dimensions: Einstein’s theories of relativity treat time as another dimension that interacts with the three dimensions of space to create a four-dimensional ‘space-time manifold’. In extreme situations (for example when objects travel at close to the speed of light, or around massive objects), different parts of the manifold can be ‘traded off’, so that for instance space dimensions become shorter while time is stretched.
This might sound strange, but it’s been proven by countless experiments. The more intriguing issue is whether there might be more dimensions than those four. Particle physicists who try to explain the various subatomic particles and fundamental forces that control the universe hope to unify all of physics with a single model known as a ‘string theory’. The idea is that all particles are tiny vibrating loops or strings of energy, ‘humming’ like violin strings, and forming different ‘harmonics’ that determine the properties they exhibit. The big catch with this neat idea is that the strings need to vibrate in either 26 dimensions (for traditional string theory), or ten (for so-called superstring theories).
The idea that there could be other dimensions beyond the ones that we’re familiar with seems mind-boggling – where are they? One idea is that they might be curled up on themselves at tiny scales, so they are invisible in normal situations. Another idea is that our four-dimensional universe is ‘afloat’ in a wider multi-dimensional cosmos that lies beyond our perceptions.
10 How galaxies hold themselves together
The discovery of galaxies other than our own Milky Way was an important one, providing us with evidence for a huge and expanding universe. One of the consequences of this discovery, however, was that galaxies appeared to be breaking the laws of physics. They are spinning so fast that the gravitational power of their observable matter is not enough to hold them together. They should be tearing themselves apart and flinging matter into space. So, why is this not the case?
The answer lies in one of the most controversial unsolved mysteries of the universe. Scientists believe that there is some hidden matter at work holding galaxies together, rendered invisible to our modern instruments but pervading the entire universe in such a way that, in fact, there is much more of it than matter itself.
As you may have guessed, we’re talking about dark matter, the mysterious invisible matter that we’ve been trying to detect for decades. We’re getting closer, but we still don’t have direct evidence it exists. Dark matter does not interact with the electromagnetic force, making detection incredibly difficult.
The matter we know of accounts for just 4% of the universe. Dark matter makes up 26% of the universe, with dark energy (a force present in the entire universe but with no gravitational effects) making up 70%. Dark matter appears to have a gravitational effect on visible matter, which would be why galaxies can hold themselves together. If we can uncover this secret, it could be the biggest discovery in modern science.
11 How black holes are linked to galaxies
In 2011, astronomers discovered a ‘naked quasar’ – a supermassive black hole with no ‘host galaxy’. Jets of material from the quasar seem to be generating stars in a neighbouring galaxy. The two objects will eventually collide and produce an ‘active galaxy’, but is this rare or the way in which all galaxies form?
12 Why light bends
Rays of light bend as they pass close to massive objects because of general relativity. Einstein’s theory explains that ‘space-time’ is distorted by massive objects. It’s as if space were a sheet with bowling balls creating dents in some areas – if you rolled a pool ball across the sheet in a straight line, its path would be deflected as it passed close to the more massive objects.
13 What is space roar?
In 2008, a NASA team’s attempt to detect the very first stars ended up discovering something even more intriguing – mystery radio waves from deep space. So far they’ve ruled out an origin in or around our own Milky Way galaxy, and there simply aren’t enough distant radio galaxies to generate such a powerful signal – so what could be causing it?
14 How was the Solar System formed?
The first scientific explanation for the origins of the Earth, Sun and other planets was put forward by Swedish philosopher Emanuel Swedenborg in 1734. Expanded on by Immanuel Kant and Pierre-Simon Laplace later in the 18th Century, this so-called ‘nebular hypothesis’ suggested that the Solar System formed from the collapse of clouds of interstellar material into a spinning, flattened disc, out of which planets grew as particles coalesced.
Laplace’s version of the theory dominated 19th Century astronomy, but was temporarily undermined in the 20th Century. In its place, astronomers put forward a range of alternatives, including theories that the planets had been captured into orbit around the Sun, that a stream of planet-forming material had been ejected from the Sun in a huge eruption, or even that the planets had been torn from our star by tides from a passing star.
All of these rival theories had their own problems, however, and in the Seventies Soviet astronomer Victor Safronov produced a widely accepted modern version of the Laplace theory, known as the Solar Nebular Disc Model. This resolved many of the problems by suggesting that the planets began their formation in an ‘accretion disc’ orbiting the newborn Sun. Here, centimetre-scale dust particles coalesced to form kilometre-sized bodies called planetesimals, which then collided to form larger bodies. Once some planetesimals grew large enough to have substantial gravity, they pulled in other material and grew rapidly in a process called ‘runaway accretion’. Finally, the planetesimals collided with each other to form full-blown ‘protoplanets’. Differences between the inner rocky planets and the outer gas giants can be explained by variations in temperature and chemistry across the original nebula, but there are still some unsolved questions – not least how the small dust particles formed into the first planetesimals.
15 How the universe began
Until about a century ago, most people were divided between the Biblical account of the universe as a few thousand years old, and the scientific evidence that the Earth and Sun were many millions, even billions, of years old. But when Edwin Hubble measured the distance to the closest galaxies in the Twenties, he also discovered that more distant galaxies are moving away from Earth more quickly. The entire universe is expanding, and pulling galaxies away from each other.
In 1930, Belgian priest Georges Lemaître pointed out that all the material in the universe must once have been concentrated in a much smaller volume. He referred to this idea of a cosmic origin as the ‘primeval atom’, but it divided the scientific establishment at the time.
Then, following breakthroughs in nuclear physics, it became clear that the extreme conditions of Lemaître’s early hot, dense universe could have spontaneously created all the matter in the universe in just the right proportions to match our observations. Fred Hoyle, an ardent supporter of the rival ‘steady state’ theory, dismissed the primeval atom as nothing but a “big bang”, and the name stuck.
The clinching evidence came in 1964, when Arno Penzias and Robert Wilson discovered an unexpected glow of microwave radiation coming from the sky. Corresponding to a temperature of just 2.7°C (36°F) above absolute zero, this Cosmic Microwave Background Radiation (CMBR) fitted perfectly with predictions for the ‘afterglow’ of the Big Bang.
Even though no other theory could explain the CMBR, there were still problems with the Big Bang – primarily the apparent ‘smoothness’ of the universe. In 1980, Alan Guth introduced inflation – the idea that a fraction of a second after the Big Bang, one small, uniform region of the infant cosmos was blown up to form the universe. Final confirmation that inflation was right came with the discovery of ‘ripples’ in the CMBR, while measurements from the Hubble Space Telescope have given the universe an age of 13.8 billion years.
16 What caused the Tunguska explosion?
On 30 June 1908, a huge explosion rocked an area in what is now Krasnoyarsk Krai, Russia, flattening an area covering around 2,000 square kilometres (770 square miles).
The cause of the event was believed to be a large meteoroid or comet fragment as big as 100 metres (330 feet) wide exploding over five kilometres (three miles) above the Earth. The huge explosion, 1,000 times more powerful than the atomic bomb dropped on Hiroshima, Japan, in World War II, is the largest recorded impact event in Earth’s history.
However, the event left few traces of an impact. Aside from the huge flattened expanse, there was no noticeable impact crater and, to date, no fragments of the meteorite have been confirmed. Therefore, the exact cause of the event remains a mystery.
Eyewitnesses living nearby described a huge flash of light and a deafening crack, but no conclusive evidence has yet been found for a meteorite impact. Other theories range from a chunk of antimatter falling from space to a mini black hole passing through the Earth. It’s unlikely that we’ll ever know for sure what happened, but further studies could help prepare us for a similar impact event in the future.
17 What are recurrent novas?
Recurrent novas are rare stars that appear dim, but occasionally flare up in brilliant eruptions. After a few weeks they subside, but they may flare back into life again years later. They are thought to be close binary stars in which a burnt-out white dwarf is pulling gas away from its companion star.
18 How the asteroid belt formed
Astronomers used to think that the large ‘gap’ between Mars and Jupiter should be home to a planet. Then, after finding asteroids there, they decided the planet must have been destroyed in some ancient cataclysm. It seems likely that there was once much more material here, but that Jupiter’s gravity prevented it from forming a planet.
19 Why is gravity so weak?
Gravity is the only force capable of making its presence felt across the vastness of intergalactic space, but compared to the other fundamental forces of physics – electromagnetism and the weak and strong nuclear forces – it’s remarkably weak. No one’s quite sure why, but it suggests a fundamental difference between gravity and the other forces.
20 Why do stars explode?
When an enormous star runs out of fuel, it can no longer support its own gravity and explodes in a huge fireball known as a supernova. We’ve been attempting to observe and study these giant cosmic explosions for decades and, while we’re finally starting to understand their cause, there are still many things we’re yet to learn.
There are two predominant types of supernova. The first, Type Ia, occur when a white dwarf accretes material from a larger stellar companion, passing a critical mass and ultimately exploding. Meanwhile Type II supernovas, also known as core collapse scenarios, are the types mentioned earlier, when a giant star runs out of fuel and collapses, resulting in a huge explosion.
Observations of supernovas and simulations in supercomputers have helped us understand what happens when these stars explode but, as of yet, we’re not entirely sure when or why they occur. For Type II supernovas in particular, the exact moment at which the explosion will occur is almost impossible to determine. For example the red supergiant Betelgeuse, over 600 light years from Earth, is known to be close to going supernova but we’re not sure when. It could explode while you’re reading this article, or it may explode in a million years’ time.
It is hoped that by studying more and more of these cosmic explosions we’ll be able to definitively determine when they will occur. For now, we can only hope that we get the chance to observe one when it does happen.