7 questions about the expansion of the Universe
What do we really know about the expansion of the Universe?
The Universe is expanding. Most of us will have heard that fact uttered numerous times throughout our lives.
Indeed, when many of us were at school, we were told that the Universe was expanding, but that eventually the gravity of all matter in the Universe would cause that expansion to slow down and reverse, leading to an end-of-Universe scenario known as the 'big crunch'.
Nowadays, we know that the expansion of the Universe isn't slowing down. It's speeding up. We call the unknown force behind this acceleration dark energy.
But how much do we actually know about the expansion of the Universe, and what can it tell us about how objects in the Universe behave?
Below are some of the most commonly asked questions about the expansion of the Universe, and the best answers we have to those questions so far.
If the Universe is infinite, how can it be expanding?
When cosmologists say the Universe is expanding, they mean something very specific.
They’re saying that the separation between the galaxies – the basic building blocks of the Universe – is growing systematically with time.
The Big Bang equations contain a ‘scale factor’ which, if it doubles in size, doubles the separation between galaxies.
It’s therefore possible for the distance between galaxies to grow, whether or not the Universe is finite or infinite.
In the case of an infinite universe, just imagine the galaxies as raisins in a rising cake – the mother of all cakes, one that extends forever in every direction!
How fast is the Universe expanding?
The rate of expansion of the Universe is expressed by a quantity called ‘the Hubble constant’.
There is always much argument over its precise value, and it is a figure that is continuously updated by new research, but the Hubble constant is about 73 kilometres per second per megaparsec (one megaparsec is just over three million lightyears).
What this means is that a galaxy that is three million lightyears further away than another is receding 73 kilometres per second faster due to the expansion of the Universe.
The expansion rate is calculated as the ratio of two quantities: the velocity of recession of celestial objects and the distance to the objects, the latter being quite difficult to determine.
However, calculating how big the Universe was at any time in its past is even more problematic.
This is because at different times, its expansion has been driven by different things:
- Inflation (first split-second)
- The radiation-pressure of photons (up until 400,000 years after the Universe’s birth)
- The gravity of matter trying to slow the expansion down (up until a few billion years ago)
- The repulsive force of mysterious dark energy (today)
The Universe may actually be infinite in extent, but we can only see the portion from which light has had time to reach us in the 13.8 billion years since the Big Bang.
After the first minute, this observable Universe was about 500 lightyears across; after a day, about 20,000 lightyears across; after a year, about 375,000 lightyears across.
Today, the observable Universe spans about 96 billion lightyears across.
This is bigger than the 27.4 billion lightyears naively expected from the age of the Universe, because the Universe expanded faster than the speed of light in its early history, which is allowed without contradicting any of Einstein’s theories.
Was the rate of the expansion of the Universe ever zero?
There was once a popular idea that the Universe had contracted down to a ‘Big Crunch’ from which it bounced into the Big Bang.
If so, then there would have been an instant – at the changeover point when contraction finished and expansion began – when the Universe’s expansion rate was zero.
However, the ‘bouncing’ Universe idea took a blow in the 1960s when Roger Penrose and Stephen Hawking proved that the Universe must have begun in a ‘singularity’.
Such an instant of infinite density and temperature breaks the laws of physics and precludes the existence of a pre-Big Bang era.
The ‘singularity theorems’ may not be the last word, however. A better ‘quantum’ theory of gravity may be free of singularities and permit a bounce.
Has the Universe's mass increased since the Big Bang?
Energy can’t be created or destroyed, only converted from one kind into another.
So for instance, a light bulb converts electrical energy into light energy and heat energy.
But does this principle of the ‘conservation of energy’ – and all energy has a mass equivalent – apply to the Universe as a whole? We think so.
However, quantum theory tells us that nature turns a blind eye if energy is created for a short enough period before vanishing again.
The Universe could therefore have arisen as a ‘quantum fluctuation’, a seed of mass-energy appearing out of nothing.
According to inflation theory, most of the mass-energy of the Universe came from the energy of the vacuum – the ultimate free lunch, as cosmologists say.
If the Universe is 13.8 billion years old, how is it 92 billion lightyears across?
The simple answer is that the Universe expanded faster than the speed of light early on.
Although it is widely believed that the speed of light is the cosmic speed limit, this is true only in Einstein’s ‘special’ theory of relativity of 1905.
Einstein later improved and extended the theory, publishing his ‘general’ theory of relativity in 1915.
It is a theory of gravity, so a year later, Einstein applied it to the biggest gravitating mass he could think of – the entire Universe.
Unfortunately, because he was wedded to the idea of a changeless, or ‘static’, Universe, he missed the ‘evolving’, or Big Bang universes, in his equations.
In such universes it is space that expands from an initial explosion, and this expanding space carries the galaxies along with it as if they are fixed to some kind of fabric.
The fabric of spacetime, because it is the backcloth of the cosmic drama and not a massive object, can expand at any rate it likes.
And, sure enough astronomers find that the Universe is 92 billion lightyears across even though it has been in existence for only 13.8 billion years.
According to the standard view, it underwent a phenomenally rapid, faster-than-light, burst of expansion during its first split-second of existence.
This inflation was driven by the repulsive gravity of an unusual state of the vacuum.
When inflation ran out of steam, the tremendous energy of this ‘false vacuum’ made matter and heated it to a ferociously high temperature. It created the hot Big Bang.
Is the expansion of the Universe speeding up?
When astronomers look at the light coming from atoms in a distant galaxy, they find that its wavelength is stretched and is said to be redshifted.
They interpret this as being because of the expansion of the Universe in the time the light has been travelling across space to the Earth.
Think of a wave painted on a balloon that is being inflated and you will get some understanding of the effect.
If the Universe is expanding at a steady rate, then light from a galaxy that has been stretched to twice its normal wavelength means that the Universe has doubled in size since the light was emitted.
But how can astronomers check whether the Universe is expanding steadily?
They use a ‘standard candle’ – a celestial object of known luminosity and distance in our cosmic backyard.
Astronomers then look for a similar object in the distant Universe.
If its redshift implies that it is twice as far away, it should be 4 times as faint; if its redshift shows it to be three times as far away, then it should be 9 times as faint, and so on.
In 1998, teams of scientists from America and Australia observed a type of standard candle in remote galaxies known as a Type Ia supernova.
They discovered that the supernovae whose redshifts implied they were twice as far away as others were less than a quarter as bright.
From this data, the scientists deduced that the Universe had expanded more than expected since the light from the supernovae had set out on the long journey to Earth.
Therefore, the Universe’s expansion had speeded up since the stars first exploded.
If the Universe is expanding, is getting to distant planets possible?
The Universe was born about 13.8 billion years ago. Consequently, we can only see stars and galaxies whose light has taken less than 13.8 billion years to reach the Earth.
This creates an ‘observable Universe’ bounded by a spherical ‘horizon’ centred on the Earth. Beyond the horizon are stars and galaxies we can’t yet see because their light is still travelling to Earth.
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The further we look into the Universe with our telescopes, the further back in time we see galaxies because of the finite speed of light.
Distant galaxies are closer in time to the Big Bang explosion, so they are moving faster.
At the distance of the horizon, they are receding at exactly the speed of light.
We can’t see objects beyond this because their light would have needed to travel faster than light, which is impossible.
Similarly, we can’t travel to them because we would have to travel faster than light, and the speed of light, as far as we know, is the cosmic speed limit for all bodies with mass.
However, if we could travel at the speed of light we would be able to reach all stars within the observable Universe.