How did the universe begin? How will it end? Why is it getting bigger? Astronomy attempts to answer some of the biggest questions possible.
Why is the sky dark?
It’s sometimes the simplest questions that stop us in our tracks. Why is the sky dark at night? The obvious answer is that the Earth spins on its axis, so we’re only bathed in light when the sun shines on our half of the planet.
But consider this: an infinite universe, with an infinite number of stars, should be bright in whichever direction you look. Imagining the sky as a series of spheres, moving outward from Earth, the number of stars in the sky would increase even as the light from each star diminishes.
Wherever you gaze, your line of sight should end at a star. The fact it doesn’t is known as Olbers’ Paradox. And although the question was discussed by the astronomer Heinrich Wilhelm Olber in 1823, it would take us around a century to find an answer that fits.
Our expanding universe
Astronomers noted that spiral galaxies – or spiral nebulae as they were known at the time – were retreating from us in the 1910s. This retreat is hard to line up with our understanding of a static universe. A Belgian priest, Georges Lemaitre, sought to explain this curiosity about an expanding model of the universe in 1927. And in 1929, Edwin Hubble proved that galaxies did seem to be flying away from us in every direction. Not only that, but the further away galaxies were, the faster they seemed to move away. In fact, the space between us and the galaxies was expanding.
Light becomes invisible to us at a certain wavelength, and we can no longer see it. As light travels further away from us, the wavelengths get stretched longer – towards the red end of visible light. This effect is known as redshift, and Hubble used it to measure how fast galaxies were flying away from us. It also explains why we don’t see the sky full of stars – some stars are so far away their light is invisible to us.
Looking back in time
According to the laws of ‘general relativity’, light travels faster than any object in the universe. It moves incredibly fast: 300,000 km/second (186,000 miles/second). However, because the universe is so vast, it still takes time for light to travel.
The light from the sun takes about 8 minutes to reach Earth. At larger scales, we measure distance in light years, the number of years light takes to travel a distance.
Light from distant stars can take billions of years to reach us. Looking at far distant stars allows us to see how they were when their light left them – to look back in time.
This is the second reason the sky is dark at night. The universe is a finite age – 13.7 billion years. A far distant light in the universe hasn’t had time to reach Earth. As the universe expands, some light will never reach us. Even the speed of light will not overtake the expansion of the universe, locking some objects out of our sight forever. Anything beyond this horizon in time and space is outside the visible universe.
The Big Bang
If the universe is expanding, it follows that it used to be smaller. And we currently believe that it was much, much smaller. There was a tiny, single, point of infinite heat and infinite density called the singularity. And then there was a big bang.
Current estimates put the age of the universe at around 13.7 billion years. It was around this point that we think this tiny, hot, dense singularity started to balloon outwards, stretching space as it went. We don’t know what could have triggered this sudden explosion. The initial expansion was incredibly fast but only lasted for a few fractions of a second, known as the inflationary epoch. During the inflationary epoch, space expanded at speeds much faster than the speed of light. After that, the expansion slowed down. But around 4 billion years ago, it seemed to speed up again.
As the universe expanded, it cooled down. A soup of matter and radiation filled the universe, conditions which would go on to form the universe we know today.
Background microwaves
If you happened to have the right telescope at hand, you could point it anywhere you wanted to in the sky and pick up a microwave background signal. In fact, you don’t even need a telescope. If you turn on an old-fashioned television, part of the static ‘snow’ that fills the screen between channels would be that same microwave background.
This cosmic microwave background (CMB) permeated the entire universe and was discovered in 1965 by astronomers Penzias and Wilson. This background radiation is remarkably uniform, with tiny variations that we need very precise telescopes to see.
It is thought to be the afterglow from the fireball of the Big Bang. The Big Bang released huge amounts of energy, and the cooling and expansion of the universe turned this energy into the CMB, which is now everywhere in the universe. The discovery of this background radiation was a massive boost for the Big Bang theory, which had predicted it many years before.
Early universe
The very early universe looked and behaved so differently from what we know now that we have no way of knowing what happened in its first moments. There are several theories about what happened in the very first milliseconds of the Big Bang, but it remains mysterious.
After that, things start to come together. Within a single second, gravity, electromagnetism, and the weak and strong nuclear forces, the forces that govern our universe, emerge from a unified super force. Then, 3 minutes after the Big Bang, it was cool enough for subatomic particles to stick together. For the next 17 minutes, the universe made atoms – mostly hydrogen and helium atoms, and a little lithium as well.
380,000 years after the Big Bang, electrons and protons were able to combine and form stable neutral atoms. Because free electrons absorb light, the universe had been opaque up to this point, but now it had become transparent. CMB radiation dates to this point.
The universe now and in the future
As the universe continued to cool, gravity could overcome heat, and structures started to take shape. During the inflationary epoch, space expanded so fast it smoothed out. But some tiny bumps in space remained.
Clumps of matter and dark matter collected around these bumps and grew into the first stars, which blinked into existence around 400 million years after the Big Bang. Galaxies and galaxy clusters followed. Star and galaxy formation both peaked long ago and are now declining with time.
For most of its history, the universe cooled and expanded, becoming thinner and less dense. Eventually, the universe was cold, vast, and sprinkled with galaxies separated by huge tracts of empty space – much like we know it today.
The expansion of space currently seems to be speeding up. As this expansion gets faster, galaxies could eventually get whipped away from view by the expanding space. Our galaxy will be alone, just as we used to think it was. And when the stars go out, only empty dark space will remain. Perhaps.
Dark energy and accelerating expansion
Although space is expanding, galaxies still have a gravitational pull on each other. At smaller distances, this gravitational pull is more than enough to overcome the expansion of space: hence the current collision course of the Milky Way and Andromeda galaxies. The gravity of all the galaxies in the universe should be acting as a brake on the expansion of the universe, slowing things down.
But this is not what’s happening. In the 1990s, 2 groups of astronomers were observing very distant type 1a supernovae. These events have a standard brightness, so we can use their luminosity to work out how far away they are. But the supernovae were fainter than expected. Eliminating all the other possibilities, the astronomers were forced to conclude they were farther away than expected.
They also concluded that the expansion of the universe was getting faster, not slower. Dark energy is the theoretical reason for this. We don’t really know anything about what dark matter is or where it comes from, but we think it makes up about 67% of the universe, and there’s enough of it to keep the universe expanding forever.
The shape of the universe
The shape of the universe is no trivial matter. Knowing the shape the universe takes could tell us a great deal about its history and its destiny. Will it expand forever or collapse back into a single point?
The crucial factor in deciding the universe’s shape is its density. If the universe is dense enough, its gravity will outmatch its expansion and curl into a ball or a hyperspace potato. This would form a closed universe. On the other hand, if the density of the universe was low enough, it would bend the other way, ending up looking something like a space saddle.
Most of our current evidence points to our gravity being at the tipping point between the two – the average density is not too high or too low. Our best current guess is that the universe is flat, or very close to being flat, and expands in every direction.
Cosmological crisis
The methods we use to measure the expansion of the universe are getting increasingly precise. But the methods we use have started to disagree with each other. This disagreement about the expansion, and therefore the age of the universe, is known as ‘the cosmological crisis’.
Astronomers can take several approaches to measure the current expansion rate of the universe, known as the Hubble constant. We can measure the distance and speeds of objects in space, such as type 1a supernovae, Cepheid variable stars, and certain red giant stars.
Another approach uses mathematical modeling and a map of the cosmic background radiation produced by the Planck satellite mission to determine the expansion rate.
The two figures produced by the differing methods don’t differ by much – only about 10%. But they both have small uncertainty values – only around 2%, so this disagreement is hard to explain. It could simply be an error in the way we’re measuring things. Or it could indicate a fundamental flaw in our understanding of the universe. The search continues for answers.
