HOW BIG IS THE UNIVERSE?
After a century of debate, we’re still not sure of the size of the universe. But the search for answers has created a new ‘Big Bang’ in our own conceptions of space and time.
Over the past century, the universe has exploded in size – according to our conception of it. Modern cosmology has redefined its dimensions from a few hundred thousand light years to a size that is a million times greater. And that’s just the part of the universe that we can see. The invisible universe is hundreds of times larger still – and might even be infinite.
It is a wonderful, a brain-staggering conception that our own stellar universe may be but one of hundreds of thousands.
ASTRONOMER HEBER CURTIS (1872-1942) in the early 1900s, when it was unclear whether the Milky Way constituted the entire universe.
Let us take a stroll around the universe! That was the invitation to an audience at the Smithsonian National Museum of Natural History in Washington, D.C. on 26 April 1920, where two astronomers were to give a lecture. From the advertised programme, few would have anticipated that the event would develop into a vigorous confrontation. But this encounter at the museum became known as ‘The Great Debate’, and it remains a legendary example of how easy it is for scientists to deliver flawed calculations when they are exploring the limits of what science can measure and observe.
The astronomers were Harlow Shapley and Heber Curtis, and the general topic of their discussion was the size of the universe.
According to Shapley, the Milky Way made up the entire universe, and the ‘spiral clouds’ that could be observed in the sky were just new solar systems under development. To Shapley, therefore, it was clear that the universe had a diameter of about 300,000 light years.
Curtis, on the other hand, considered the spiral clouds to be independent galaxies which were located far beyond the stars of the Milky Way, so that he believed the universe to stretch much further than our own galaxy, which he considered far smaller, with a diameter of some 30,000 light years.
Shapley was slightly closer with his estimate of the Milky Way’s size – the diameter is now estimated to be 100,000-150,000 light years. But he was wrong regarding the spiral clouds. Curtis, on the other hand, had underestimated the size of the Milky Way, but he was right about the big picture, and the existence of far more distant independent galaxies.
The idea that the Milky Way might be only one of many galaxies was both controversial and bold, as Curtis himself admitted, saying: “It is certainly a wonderful, a brain-staggering conception, that our own stellar universe may be but one of hundreds of thousands of similar universes.”
If it seems surprising today that such a fundamental fact about the universe was still under discussion a mere 100 years ago, it demonstrates the leaps made in cosmology since then. Besides, the two astronomers were facing a challenge with which modern cosmologists are still struggling. It is simply very difficult to measure distances in the universe.
Too big to comprehend
The universe has been growing in our conception of it throughout the history of astronomy. The estimation of distances to remote objects has been particularly marked by gross underestimation. Roman-Egyptian mathematician and astronomer Ptolemy (100-170 AD) had a surprisingly good idea of nearer dimensions in comparative terms, such as stating the Moon’s distance compared to Earth’s size. He calculated that the distance to the Moon was 29.5 times Earth’s diameter – very close to the modern figure of 30.2 times Earth’s diameter.
The Sun's distance was more difficult. Ptolemy came up with a value around a twentieth of the correct one. And for the stars, he was simply unable to fathom the possibilities. He estimated them to be about 10,000 Earth diameters away. The real distance to the closest star, Alpha Centauri, is 6,455,555,555 Earth diameters. It didn't help that Ptolemy also underestimated Earth’s diameter. His entire universe could have fitted inside a region corresponding to Earth’s orbit around the Sun.
More accurate measurements had to wait for astronomers such as Tycho Brahe and Johannes Kepler in the 1500s and 1600s. But their view was still limited by an increasing inaccuracy in their methods for calculating longer distances. The only method available to them was the parallel axis theorem, drawing sight lines towards a star at six-month intervals, and subsequently calculating the distance to the star based on the angle between the sight lines and the diameter of Earth’s orbit around the Sun. The method is good, but it requires more accurate instruments than were available to Brahe and Kepler, who couldn’t reliably calculate distances to even the closest stars. And the astronomers of their day did not conceive anything more remote than the stars in the night sky.
Isaac Newton didn't either, when he published his major work Philosophiae Naturalis Principia Mathemat
ica in 1687. But inspired by his falling apple, Newton did change his view of the universe, by defining gravity as its controlling power. The same rules apply to the motions of all heavenly bodies, he proclaimed, and they all exist in a uniform space. This founded what is known as ‘the cosmological principle’ in modern cosmology, that when viewed on a sufficiently large scale, the properties of the universe are the same for all observers.
The cosmological principle
Imagine that you are standing on the surface of a huge inflated spherical balloon. Look in any direction along its surface, and it looks the same.
That is the essence of the cosmological principle, and it usefully deals with any idea that there is something special about our location in the universe.
The principle involves two assumptions. One is that on a grand scale, the universe is homogenous, roughly the same and with the same qualities no matter where we might be located. The other is that it is isotropic, meaning that things look the same in any direction.
The cosmological principle also implies that the universe has no specific centre – or that all places are an equivalent centre, as is the case when we’re standing on the surface of that spherical balloon.
Like Newton, Albert Einstein supported the cosmological principle, but his general relativity theory from 1915 also provided cosmologists with a new way of viewing the universe. Einstein united time with the three spatial dimensions to produce his 4D space-time, and he could use his equations to calculate models for how the
universe must look on a grander scale. But he soon encountered a problem. When he used his equations on the entire universe, he didn't get the expected result – either the equations showed that gravity would quickly make the universe collapse, which clearly hadn't happened, or the equations showed that the universe was growing, which contradicted thinking at the time that the universe was static, and of constant size.
Einstein solved the problem in 1917 by introducing a constant into the equations, which later became known as ‘the cosmological constant’. It was a step that he would later bitterly regret, reportedly describing it as “the biggest blunder of my life”.
The universe develops growing pains
Einstein’s biggest blunder was unmasked by a very large telescope which was put into service in that same year of 1917. With its 2.5-metre mirror, the Hooker telescope at California's Mount Wilson Observatory had the sharpest vision of its day. Together with astronomer Edwin Hubble, it was about to revolutionise our understanding of the universe.
Hubble began to work for the observatory in 1919, and his access to the telescope allowed him to study the spiral clouds about which Shapley and Curtis had their ‘great debate’ in 1920. Hubble was searching for a very specific type of star in the clouds – cepheids, which exhibit brightness that varies according to a specific rhythm, with a close connection between a star’s rhythm and its light intensity. By observing a star’s rhythm, astronomers can calculate how much light it emits, and then can subsequently estimate how far away it is by applying what they know about how light intensity is reduced across a distance. Hubble discovered cepheids in several spiral clouds, including Andromeda, and in 1924 he could assert with confidence that there were such things as ‘foreign galaxies’ far beyond the other stars of the night sky.
In spite of his epoch-making realisation that our own galaxy is only one among countless others in an incredibly vast universe, Hubble humbly considered his discovery to be just another step within the story of mankind’s reaching for the stars, writing that: “The history of astronomy is a history of receding horizons.”
Hubble continued to look deeper into space, using the large telescope to zoom in on even remoter galaxies and then analysing their light. This led to another breakthrough in 1929, when Hubble discovered the ‘red shift’, where the further away a galaxy is, the redder is its light, a phenomenon caused by the light waves of an object being stretched if it is moving away from us.
Again Hubble underestimated the importance of his discovery. He was a practical astronomer more than a theoretical cosmologist, and he did not consider the consequences if red shift were to be a quality of the entire universe. The red shift of light from remote galaxies could only mean that they are moving away from us and, because the cosmological principle implies we are not in a special central position, that
they are also all moving away from each other. Einstein quickly realised the implication that the universe was not static, but expanding. So there had been no need for the cosmological constant he had introduced into his equations 12 years previously. His ‘blunder’ was revealed.
The implications of red shift went further. The breakthrough supported the ideas of Belgian priest and astronomer Georges Lemaître who, two years previously in 1927, had introduced the conception of a universe that had expanded – and hence had not always existed. Hubble’s observations were perfectly consistent with Lemaître's idea, where the galaxies were moving away from each other as if painted on the surface of a balloon that is gradually inflated. And if this were true, it would also be possible to calculate backwards to a time when the entire universe was united at one point. Lemaître imagined that the universe was born in the explosion of a “primaeval atom”, and had been expanding ever since.
In the following years, Lemaître's ideas were adopted by several other astronomers, developing into what is now know as the ‘Big Bang’ model. But this was far from being widely accepted. The very name ‘Big Bang’ was invented by one of the theory’s main opponents, British astronomer Fred Hoyle. He used the term rather scornfully during a 1949 radio programme in which he argued in favour of his own alternative, the Steady State theory. Together with other proponents of that theory, Hoyle believed that the universe was expanding, but not changing, since matter would be constantly produced in step with the expansion, so that the universe’s density would remain the same. This addition of new matter has never been observed, but according to the theory’s proponents, it probably wouldn’t be, because very little would be required to maintain the density. Calculations show that it could be sufficiently achieved through the generation of a quantity of matter corresponding to a single hydrogen atom per cubic metre per billion years – hence it is not strange that we might not observe this happening. And furthermore, the Steady State theory not only satisfies the cosmological principle, it achieves “the perfect cosmological principle” by being homogenous over time as well as space, with the continuous formation of matter ensuring that the universe remains homogenous in all spatial directions and throughout its development. According to the Steady State theory, the universe is infinite as regards both time and space, and unlike the Big Bang model, it does not require that the universe had a ‘beginning’.
Over the next decade and a half these two opposing theories were strongly contested one against the other, until an accidental discovery in 1964 provided a major evidential boost for one of them.
What’s that noise?
Two radio astronomers, Arno Penzias and Robert Wilson, were struggling with a 15-metre-long horn antenna in New Jersey. They had received permission to re-use the antenna, originally designed to communicate with a huge metallic balloon satellite called Echo, to conduct a survey of radio signals from space. But no matter what they did, their measurements were interrupted by a continuous and irritating background noise. Penzias and Wilson aimed the antenna away from New York to eliminate man-made radio sources. The noise continued. They discovered that the antenna’s horn was full of pigeon and bat excrement. They scraped it all out, along with a few pigeon nests, cleaning and polishing every component. They resumed work with a clean antenna. The noise remained. After a full year they concluded that a microwave background was present for which they could not account.
Then in 1964 at an astronomy conference, Penzias learned that Robert Dicke and James Peebles from Princeton University were predicting that light from the ‘Big Bang’, if that theory were correct, might today be detectable as microwaves, an idea which had been presented even earlier by another American, Ralph Alpher, that there should be very weak, uniform radiation from all directions, which dated back from the birth of the universe.
Penzias and Wilson contacted Dicke, and together they concluded that their irritating noise was indeed exactly this predicted cosmic background radiation. In 1965, the two radio astronomers and Dicke published their results and interpretations of the data.
Einstein shapes the universe
The cosmic background radiation confirmed both the Big Bang model and the cosmological principle stating that on the grand scale, the universe is homogenous and isotropic. The radiation is almost uniform no matter in which direction it is measured, and it would remain so even if Earth had been located in a different place.
The cosmic background radiation was released when the universe was only 380,000 years old. The universe had sufficiently expanded and cooled for electrons and protons to unite into atoms. This meant that radiation in the shape of photons, which was previously halted by free electrons, could now travel freely through the universe – astronomers talk about the universe becoming ‘transparent’; indeed this is thought to be the limit determining how far back in time it is possible to see.
The discovery of background radiation thereby gave the universe a history of development, with the ‘Big Bang’ model defining when the universe was born and how it has subsequently changed. And it can even reveal the shape of the universe.
When Einstein published his general relativity theory, several astronomers began to use his equations to calculate the geometry of the universe as a whole. One of them was Alexander Friedmann of Russia. In 1922, he introduced models for how the universe might appear if it were to satisfy both Einstein’s equations and the cosmological principle.
The history of astronomy is a history of receding horizons. Knowledge has spread in successive waves. ASTRONOMER EDWIN HUBBLE (1889-1953) modestly places his revolutionary discovery of an expanding universe within the context of historical astronomy.
With these constraints, the universe must bend in the same way, no matter where we are. And this can only happen in three different ways.
First option: the universe could be closed, like the surface of the balloon we imagined earlier. A closed universe has a limited extent. If we send two parallel light beams out into a closed universe, they will meet at some point, as do longitude lines on Earth at the poles. Second option: the universe bends in the opposite way, resulting in an open universe with a saddle-like shape. Here the two light beams will never meet, moving ever further away from each other. Such an open universe has no limits: infinite in all directions. The third option is a flat universe, in which the two light beams remain parallel. Like the open universe, the flat universe is infinite. Since the discovery of background radiation, it has been mapped out several times, with the COBE, WMAP, and Planck satellites providing ever more detailed recordings of the radiation, And analyses indicate that the universe is flat – or at least close to being flat.
To astronomers, the shape of the universe was quite important, as it was closely linked with the likely future of the universe. A closed shape would mean that gravity would at some point overcome the expansion of the universe, which would have to contract and end up in a collapse, a ‘Big Crunch’. But that scenario was revised radically around 2000, and once again it was due to astronomers using a new method that could reach deeper into the universe.
Bigger and faster
Astronomers love a supernova, and one special type in particular, known as ‘Ia’. This type of supernova originates in a special way, so that its light can be used to measure distances.
An Ia supernova forms from a double star system, in which one of the stars is a white dwarf. If the two stars orbit each other closely, the white dwarf will gradually absorb matter from its mate, until it reaches a specific critical mass. Then it explodes into a supernova, producing light so intense that it outshines the light of all other stars in its galaxy. As astronomers know the critical mass, they also know the supernova’s absolute brightness, and so they can calculate the distance to it in the same way as Hubble could with the cepheids. But supernovas are very much brighter than cepheids, and so they can be observed over much longer distances.
In 1998, American astrophysicist Saul Perlmutter and his colleagues from the Supernova Cosmology Project began to look for Ia supernovas in very remote galaxies. They measured the light from them and subsequently calculated their distances. And also, just like Hubble, they studied how much the light had been red-shifted, to establish how fast the galaxies are moving away from us.
The light from such very remote galaxies has not only travelled further than the light from galaxies that are closer, it has taken longer to do so. We are looking back into time, so the light can tell us how fast the universe was expanding billions of years ago. And surprisingly, it turned out that the speed of the more remote galaxies was apparently much lower than it should be, according to the logic discovered by Hubble. There could be only one explanation: the galaxies of the universe were moving more slowly away from each other billions of years ago than they do now. In other words, the expansion of the universe is accelerating.
“What we were seeing was a little bit like throwing the apple up in the air and seeing it blast off into space,” Saul Perlmutter later said about the realisation, referring to the the apple that made Newton realise the nature of gravity. At the same time as Saul Perlmutter’s breakthrough, a rival team of scientists headed by Adam Riess arrived at the same results, and both Perlmutter and Riess were awarded the 2011 Nobel Prize for their contributions. Yet their result indicated the opposite of what was expected. If nothing but gravity were at play, then the expansion of the universe would not accelerate, but rather slow down with the age of the universe.
An accelerating expansion of the universe indicates that there is a force that has the opposite effect of gravity, and is significant enough to overcome it. But what is this force, and where might it be coming from?
The answer is made particularly hard to find because in our own galaxy and even in the galaxies close to ours, we do not observe the effect of the opposing force, because gravity is much stronger at such distances. But on a grand cosmological scale, the unknown force plays a vital role – indeed it implies we are unaware of some 70% of the total of matter and energy existing throughout the universe.
In 1998, the unknown force was named “dark energy” by American cosmologist Michael Turner, as it does not interact with electromagnetic radiation such as light. The name is doubly appropriate, because scientists are completely in the dark when it comes to understanding what makes up this mysterious force.
Space fills up with dark energy
There are clues, yet these can seem equally baffling in themselves. One of the oddest characteristics of dark energy is that there seems to be ever more of it. The dark energy makes the universe expand, so the average density of the matter in the universe would, logically, become ever lower. But the density of dark energy is constant, so in a growing universe, the quantity of it must grow proportionally. This reinforces the acceleration – dark energy produces expansion, allowing room for new dark energy, which causes more expansion... et cetera.
Although scientists do not know the nature of dark energy, they can calculate its effect, and it turns out that mathematically it is very much in keeping with the cosmological constant that Einstein introduced in his equations in 1917. It turns out that the idea he himself considered to be a blunder was not all that stupid after all.
The cosmological constant can be understood as a measure of vacuum energy. It is the minimum energy – also known as zero-point energy – that is present in a perfect vacuum in space. The problem here is a huge difference between the value that scientists calculate from theory and the value that is consistent with observations. This is currently considered one of the major unsolved mysteries of science.
However, this does not make astronomers doubt the Big Bang theory. All sorts of other observations are so consistent with the Big Bang that it is now widely accepted as the model of our universe’s history. It also provides us with an idea of the universe’s size.
The light from the most remote galaxies that we can see in the universe is red-shifted so much that astronomers calculate it to be about 13.8 billion years old. This means that the ‘transparent’ universe is about the same age. But that doesn’t mean that these galaxies are located 13.8 billion light years away from us. If the universe has expanded while the light from the galaxies travelled towards us, then they are now much further away than when they emitted the light. Indeed, the remotest galaxies from which light reaches us are now located 46.1 billion light years away. And if this is true in any direction, then what astronomers call “the visible universe” has a diameter of 92.2 billion light years.
But what about “the invisible universe” – the part of the universe that is even further away and which we cannot see? Here, cosmologists become more careful – but a few of them have already tried to come up with a theory.
Invisible, infinite, and faster than light?
In 2016, scientists from the University of Oxford set out to calculate the dimensions of the invisible universe. They collected all the distance measurements that they could get concerning all object types and fed them into a complex computer model, programming the computer to calculate all possible scenarios in which the measurements could make sense, including the likelihood of the distance measurements being consistent with different universe curvatures, and what these would mean to the general geometry of the universe.
The computer’s most probable answers were consistent with a universe that is almost flat. A completely flat universe would mean that its extent is infinite, if the cosmological principle holds, since this says that the universe is the same in any direction, and in a flat universe this can only be so if it is infinite. But we could also imagine an invisible universe that is so big that our visible universe makes up only a very small part of it – as if we drew a small circle on the surface of a huge balloon. If so, we might experience a visible universe that was almost flat, although it was really bending slightly.
Astronomers’ cautious interpretation of the Oxford results is that the invisible universe is at least 251 times as big as the visible universe. That gives it a diameter of
What we were seeing was throwing the apple up in the air and seeing it blast off into space. ASTROPHYSICIST SAUL PERLMUTTER, after he and his colleagues made the amazing discovery that the universe's expansion is accelerating.
23,343 billion light years. But the scientists emphasise that it might easily be even larger. and perhaps even infinite.
No matter how big the universe, we can be sure of one thing: it is growing even bigger. For the past seven billion years, dark energy has outcompeted gravity in the universe, and the expansion has accelerated. This will continue; indeed dark energy will dominate the universe ever more, so that the speed of expansion will gradually grow, becoming much higher than it is today, the galaxies of the universe moving ever faster away from each other.
They can even move away from each other faster than the speed of light, which is otherwise the absolute and impassable speed limit of the universe. This apparent anomaly is possible because the distance to the galaxy increases not only because the galaxy moves away from us, but because the space itself also expands. The remotest places of the visible universe, located 46.1 billion light years away, are now moving away from us at 10 times the speed of light, due to the expansion of the universe. Any light that might come from these regions in the future will never reach us.
That is also how it will go with galaxies which are now closer to us. Eventually they will be travelling so fast away from us that we will no longer see their light, and in the far future it will not be possible to observe galaxies other our own (assuming we are still around to observe such things).
So, we are fortunate to live in an era during which we have such an exciting and expansive view of foreign spiral galaxies, elliptical galaxies, quasars and other exotic astronomical phenomena. If we were living in a later era, everything that we observed would be contained within our own galaxy, and we would be likely to conclude that our own Milky Way constitutes the entire universe – just as astronomers thought 100 years ago.