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Return To Infinity!

Les Reid wonders whether the universe might turn out to be infinite after all.

Scientists recently managed to capture a direct image of a black hole for the first time. M87 is a supermassive black hole fifty five million light years away which is larger than our entire Solar System. Twenty years ago, black holes seemed more science fiction than science fact. Now we have good reason to think that there is a black hole at the centre of every galaxy, including our own, the Milky Way. So, not only are they real, they’re commonplace! Now the question is – will black holes swallow up Big Bang cosmology?

Although his name is prominent in cosmology, Edwin Hubble did not invent the idea of the expanding universe. Credit for originating that idea belongs to another American, Vesto Slipher, who suggested in 1912 that the observed red shift in the spectrum of light from distant galaxies is caused by their recession. According to him, the light from distant galaxies has been stretched into the red end of the spectrum because its source is moving away from us. He compared the red shift of light to the Doppler effect observed with sound waves, where a sound source moving away from the listener (say, a police siren) produces a deeper tone than it would if it were stationary. Hubble developed Slipher’s idea by collating and analysing the light from many galaxies. He organised his results in a graph, published in 1929, which revealed a pattern: the more distant a galaxy, the more red shift in its light, and therefore the higher its velocity away from us. That relationship between distance and velocity is known as Hubble’s Law. However, though the general pattern has been confirmed in all observations, the exact ratio between distance and velocity, called the Hubble Constant, has proved very difficult to specify, and that failure has led to an impasse in cosmological theory.

The idea that distant galaxies are receding from us led to certain conclusions in cosmology. First, if all galaxies are now receding, we can theoretically wind the clock back to find a common starting point. Second, if we observe them receding in all directions from us, then this common starting point must have been where we are now. However, the second conclusion is unacceptable because it treats Earth as a very special place – the centre of the universe. But how could we avoid this elitism?

The answer was to reinterpret recession not as physical motion in a fixed Newtonian space, but as the expansion of space itself in a mutable Einsteinian cosmos. The difference is that the cosmos as Newton understood it is infinite in all directions of space: the dimension of time is also infinite, both back into the past and forward into the future. A Newtonian rocket blasted into space and maintaining a straight line would travel forever. By contrast, the cosmos according to Einstein is finite, and shaped by the matter which it contains. Matter curves spacetime, and the expansion of matter causes spacetime to expand with it. Due to spacetime’s curvature, an Einsteinian rocket blasted into space would eventually return to its starting point. Hubble and fellow Big Bang cosmologists concluded that rather than the galaxies themselves moving away from us, the spaces between the galaxies are expanding, and therefore that the universe itself is expanding. The expansion causes red shift because light has a constant velocity and so the light we observe is being stretched to fill the increased space. Because the expansion is happening everywhere, the same red shift and the same apparent recession would be observed, no matter where in the universe the observations were made. In other words, Planet Earth is not at the centre of the universe any more than anywhere else is.

Reflecting (on) the Infinite © Katie Bell 2020

Return to the Source

The theory of an expanding universe does imply that if we wind the clock back then spacetime itself shrinks back to a condensed source. Everything emerges from that primordial egg: space, time, matter, energy, the lot – from which our present universe is the aftermath, still expanding and cooling.

Big Bang cosmology is counterintuitive for some because it posits a starting point for the cosmos. We have long been used to the idea that stars form out of gas and dust due to gravity, ignite as nuclear reactions, and eventually burn out, leaving gas and dust which will be recycled into other stars. The same may be true of galaxies, also forming out of gas and dust and then swirling inwards until they eventually explode. Such processes could be endless, and in contrast to Big Bang cosmology, require no starting point. That view was the basis of Steady State cosmologies, as propounded by Fritz Zwicky, Fred Hoyle, and others, from the 1930s. Zwicky challenged the assumption that red shift is caused by recession. He proposed that it is caused by the distance light has travelled; so his theory was dubbed ‘tired light’. This successfully explained why the most distant galaxies have the greatest red shift – the light from them has travelled the greatest distance. However, Zwicky could not arrive at a satisfactory mathematical framework for his theory, so it lost out to the Big Bang regardless.

Edwin Hubble died in 1953. Astronomy has taken great strides since then – some of them upon the Moon! Quasars and pulsars have been discovered. Satellites have been placed in orbit round the Earth, including the Hubble Space Telescope; and exploratory craft have sent back detailed pictures of other planets and their moons. The Voyager spacecraft, launched in the 1970s, have exited the Solar System, and are now travelling through interstellar space. Mapping of the galaxies has revealed large scale structures involving huge numbers of galaxies; for example, the Great Wall, discovered in 1989.

One discovery cosmologists thought very significant was made in 1964. This was a low level radio hiss observed in all directions in the universe: the Cosmic Microwave Background Radiation (CMBR). The existence of the CMBR had been predicted by Big Bang cosmology, and so its discovery was treated as confirmation of the theory. It is said to be the afterglow of the initial cosmic explosion, when a blast of light preceded the ejection of matter, hence its uniform presence throughout the cosmos.

However, doubts later crept in, arising from that very ubiquity. These doubts are known as ‘the Horizon Problem’. The concern is that as the universe expanded, regions of space which were diametrically opposite very soon passed the point at which information could be exchanged at the speed of light, and were therefore absolutely cut off from each other. But if the various regions of the cosmos were so isolated, how come the background radiation is identical in all directions? For an analogy, think of life emerging on another planet completely cut off from Earth. How likely is it that the life forms there will be identical with those found on Earth? Not likely at all.

The currently accepted solution to the Horizon Problem is to posit, prior to the universe assuming its normal expansion rate, an ‘inflationary’ period of very rapid expansion early enough after the Big Bang for all parts of the universe to be within the necessary information horizon for a uniform release of the CMBR. If that solution sounds rather ad hoc, there is an alternative take, based on the findings of the COBE satellite of the 1970s – namely, that the CMBR is not uniform after all. The COBE results have been refined into a map showing the variations in CMBR across the sky. Surprisingly the new ‘contour’ map is treated as evidence in favour of Big Bang cosmology, just as the original uniformity of the CMBR was. It seems the Big Bang Theory cannot lose!

But another problem has cropped up. The red shift of distant galaxies now appears to show that they are not merely receding but accelerating away from us. Big Bang cosmology being otherwise unable explain this, it is now proposed that there is a vast amount of otherwise undetectable ‘dark energy’, acting as a repulsive force to push galaxies apart. As the Big Bang acquires more bells and whistles to keep the maths right, perhaps a cosmological rethink might be timely.

Unpacking Space-Time © Krisztian Kotai 2020

Eclipsed by Dark Matter

A dramatic development in astronomy has been the realisation that what cannot be seen is far more abundant than what we can see. For instance, we cannot usually see a black hole because its gravity is so strong that no light can escape it (hence the name). We have to infer its presence from its effect on neighbouring bodies, usually stars.

For many years black holes were thought of as merely a fanciful possibility. But studies have since found evidence for a black hole at the centre of every galaxy, including our own Milky Way. Black holes were also originally thought to be in a fixed state, but theoretical work by Stephen Hawking has shown that they leak radiation, and eventually evaporate. The largest black holes are called ‘supermassive’, and they are vast. M87, for example, is forty billion kilometres in diameter and therefore larger than the Solar System. It has a mass six-and-a-half billion times that of the Sun.

Like black holes, the existence of dark matter was proposed long before it was accepted. In 1933, Zwicky was studying the Coma cluster of galaxies. He reasoned that the best explanation for what he observed was that there was more ‘dark matter’ (his term) present than visible matter. He meant by this matter which does not reveal its presence except through its gravitational effects.

The idea lay dormant for forty years. Then in 1973, Jeremiah Ostriker and James Peebles found that their computer simulation of galaxy-formation would only produce the spiral arm shape common amongst galaxies if they included a large amount of dark matter. Likewise, when Vera Rubin and Kent Ford studied the rotational speeds of galaxies, they concluded that the rotations could not be explained without large quantities of dark matter. Astronomers now think that there is more dark matter in the universe than ‘normal’ (baryonic) matter. What exactly it is, though, is still a subject for debate.

The recent discoveries concerning black holes and dark matter could be said to amount to a change of the premises on which the argument for Big Bang cosmology was based: there is much more matter in the universe than previously thought, and that means much more gravity, too. These discoveries should change our conception of the journey light takes when it travels from another galaxy to our own. First, light is pulled back by the gravity of the galaxy from which it departs; then it travels across intervening gravitational fields; finally, it is pulled inwards by the gravity of the Milky Way. So perhaps it is gravity in all its forms which causes red shift, not recession, in which case we have no need to assume that the universe is expanding. Perhaps Slipher jumped to the wrong conclusion in 1912, and Hubble should have looked before he leapt after him.

It is intriguing to think that we may be at the point of transition from one scientific paradigm to another. Just as Ptolemaic cosmology gave way to the Copernican revolution, so Big Bang cosmology may be about to give way to a cosmology based on gravity and dark matter. It would be an upheaval of similar significance. Big Bang Theory has given us a universe with a starting point and a singular history, whereas the new cosmology might imply a return to infinity – a cosmos with neither beginning nor end, eternally recycling matter and energy as galaxies and galactic structures coalesce, persist, explode, or evaporate, and disperse, only to coalesce again elsewhere.

The Big Bang has been standard cosmology for so long that it has acquired the appearance of established fact. But science keeps moving onwards, discovering new facts and revising its theories in the light of those discoveries. Like the Titanic, which was labelled ‘unsinkable’, the Big Bang may have struck a supermassive dark iceberg. We await further developments with interest.

Les Reid teaches a course on Humanism as part of the Edinburgh City Council adult education programme.