The observable universe is currently estimated at 92 billion light years in diameter. This is based on observations by the European Space Agency’s Planck space mission in 2013. Scientists know the universe is expanding, and that it is 13.82 billion years old. By extrapolating the rate of expansion from the point of the big bang (13.82 billion light years away) to the present, they can estimate how far the universe has expanded from the centre. The answer is 46 billion light years (giving a diameter of 92 billion light years). You would have to travel about 435 billion trillion kilometres to reach the edge of the universe in any direction—itself a moving target since the universe is still expanding (and faster with time!). That is why there are three certainties in life: death, taxes, and the fact no one will ever reach the edge of the universe.
Two hundred years ago they thought the universe was a crystal sphere speckled with fixed stars. And a hundred years ago they thought the Milky Way was the edge of the universe. Yet we now know that there are at least as many galaxies in the universe as there are stars in the Milky Way.
George Lamaître (1894 – 1966) predicted in 1927 that all galaxies should be moving away from each other, and the further away a galaxy was the greater its speed of recession from the Milky Way. A few years later he went on to hypothesise that this implied an expanding universe, and that it had been hotter and denser in the past having originated from, what he termed, a “primeval atom”. In 1922 Alexander Friedmann (1888 – 1925) had already demonstrated that an expanding universe was consistent with Einstein’s theory of general relativity.
In 1948 physicists Ralph Alpher and Robert Herman said that if Lamaître was right about his primeval atom then this would have left a detectable afterglow. In 1964 Arno Penzias and Robert Wilson discovered radio noise coming from all over the sky, which matched perfectly the 1948 prediction that a big bang would leave behind an afterglow of primordial light. This radio noise was termed Cosmic Microwave Background Radiation (CMBR).
In 2013 the European Space Agency’s Planck Satellite mapped the CMBR, which showed the universe as it was when just 380 000 years old. Until then the temperatures prevented atomic nuclei from combining with electrons to make atoms. Only when the cosmic soup cooled sufficiently could those atoms finally form, and it was at this point that light could escape from the chaos of the big bang and create the first available picture.
The average temperature of the CMBR is currently 2.73°Kelvin. That is, 2.73° above absolute zero, equivalent in centigrade to -270.4°. Absolute zero is the theoretical temperature at which matter is so compressed that it no longer exists; adding credence to the belief that all matter in the universe was once compressed into an almost infinitesimal point just before the big bang.
Pure energy at the time of the big bang also produced the most dangerous material in existence – anti-matter (which has a negative charge). If they are equal then matter and anti-matter will annihilate each other and end up as radiation. There will be nothing. It is only because matter was one million and one, while anti-matter was one million: that the extra one brought existence into being. After 380 000 years the temperature had fallen to about 2 700°C (3 000°Kelvin) and photons slowed down enough to be captivated by protons and become infra-red photons. The first neutral atoms began to form, and the universe started to become transparent. The first element to be created was hydrogen, and within three minutes helium and lithium were created. Hydrogen is needed for creating stars, but this did not happen for many millions of years. Research is ongoing, so we are left with an estimate of between 200 million and 500 million years later when, for the first time, objects “switched on”, and stars began to form, producing the heavier elements of nitrogen, carbon and oxygen.
Some eight billion years later gravity began to draw in dust so that, by some nine billion years after the big bang, our sun and Earth were created.
Vera Rubin (1928 – 2016) determined in 1975 that once all the interstellar gas and dust was taken into account the galaxies seemed to behave as if they weighed about six times more than their visible matter (about 85% of the apparent mass was missing). The answer was “Dark Matter”. The reason we do not see dark matter is precisely because it barely emits radiation. In other words, dark matter particles do not absorb, scatter or emit light. The only way to detect dark matter is through the effects of its gravity on physically seen objects.
“Baryonic matter” is a term used by astronomers to refer to all objects made of normal atomic matter (neutrons, protons and electrons). Technically, baryonic matter should include matter composed of baryons, which are neutrons, protons and all the objects composed of them, that is, atomic nuclei; but exclude things such as electrons and neutrinos, which are actually leptons. For their purposes, however, astronomers include electrons in their general use of the term baryonic, arguing that electrons are extremely insignificant in the big scheme of things, accounting for less than 0.0005 of the mass of visible matter. Neutrinos, however, are correctly taken as non-baryonic by astronomers.
The leading theory is that dark matter is non-baryonic—what physicists term “weakly interacting massive particles”, or Wimps. It is theorised they even pass through normal baryonic matter as if it did not exist.
But there is a counter theory that suggests dark matter is in fact baryonic, what they term “massive astronomical objects”, or Machos. In concept, Machos would be compact accumulations of normal matter (baryonic) that orbits galaxy halos. These might include black holes, dead neutron stars, white dwarf stars that have cooled down, and even stray planets. Because of their materials they also do not emit light radiation, so their mass remains undetected.
So far modern telescopic observations have not as yet identified sufficient amounts of this dead matter and star remnants, suggesting Wimps are preferred over the Machos theory. Either way—baryonic or non-baryonic—dark matter permeates the cosmos. Incidentally it makes up super black holes that live at the centre of galaxies.
In summary, dark matter explains the gravitational forces evident in spinning galaxies, and why they spin faster than is indicated by their visible mass. Without that gravity the hot gases would disperse, and the galaxy would break up. Of course, all this assumes that our current understanding of gravity is correct.
Dark Energy is something we don’t see either. But we know it is there because it is the force that is driving the expansion of the universe and doing so at an ever-accelerating rate. Astronomers currently estimate that the universe is expanding at 71 kilometres a second.
Dark energy was first observed in 1998 and is now asserted as the main component of the modern universe. It accounts for 68.3% of all energy in the cosmos. Even so the energy content of each cubic kilometre of space is extremely tiny. But over the massive breadth of the universe the minute forces amount to massive energies.
One can consider dark energy as a sort of anti-gravity, or as a type of repellent. It is pushing everything apart at ever faster rates and separating galaxies.
The sum total of all the dark matter of the universe is not sufficient to satisfy the theories of its evolution. Dark energy is proposed as an essential part of the equation that provides the answer. It is put forward as the force that is opposing gravity and causing matter to fly apart. In a sense dark energy is the master of the universe. Though its exact nature remains a “matter” of grave speculation.
|Mass-Energy content of the universe|
|Baryonic (visible) matter||4.9%|
The composition of the universe after the big bang, but before the formation of stars, was hydrogen 76%, helium 24% and trace amounts of deuterium, tritium and lithium. Hydrogen collects and clumps together under its own gravity. We call the result “stars”.
The first massive stars emerged between 200 million and 500 million years after the big bang. In an endless cycle of death and rebirth stars reprocessed hydrogen and helium into more complex heavier elements and dispersed these new chemical elements into space and into other collapsing protogalactic clumps. Stellar processes reduced the amount of hydrogen to 74%, and helium to 23%, evolving new elements in the form of oxygen 1%, carbon 0.5%, neon 0.5%, iron 0.1%, nitrogen 0.1% and traces of other elements. Carbon, oxygen, silicon and iron were formed through nuclear fusion in the cores of stars. And when these stars in turn went through their violent death throws still heavier elements were formed including barium and lead. Over billions of years this process allowed for the accretion of rocky planets.
How did the universe start? Was the “Big Bang” the beginning? Or was it just another restart of an endless cycle? If it was the beginning, is there an end? How does it end? No one really knows. Theories abound. There’s the “Big Rip”, “The Big Chill”, “The Big Crunch”, take your pick. Whether there is an end and when it happens is certainly nothing to worry about.
The most cogent theory to date is that a single entity created the universe some 13.82 billion years ago, that it will reach its apex in another seven billion hence. That’s 21 billion years after the big bang. The theory continues that, at this point, like a tennis ball thrown up into the sky and reaching its apex, the expanding universe pauses momentarily before reversing the process and starts to shrink, leading to the “big crunch” in another 21 billion years where matter disassembles itself back into the original infinitesimal point. That would give the universe a life cycle of 42 billion years.
But this is the good news. Some physicists say that if this does not happen then the universe will continue expanding at an ever-increasing rate. The process will reach a climax in 50 billion years when the whole universe will have been pulled apart and everything will be reduced to individual atoms. Everything will be destroyed in a massive cold chill. And the culprit is dark energy, “sucking out” the universe into infinitesimal nothingness.
So will the universe disappear? Well, in a sense it might. If the universe is expanding at an ever-accelerating rate, will it exceed the speed of light? Indeed, is it possible to exceed the speed of light or is there a speed limit? If there is a speed limit, then is this the point at which dark energy fails in its catastrophic quest to pull everything apart and then the universe will begin to shrink.
Speculation itself is fascinating.
Then again if there is no limit to the accelerating speed of an expanding universe then, when it exceeds the speed of light, it would disappear. We cannot see dark matter, so the theory goes, because it does not emit light. In the same way, if an expanding universe exceeds the speed of light it will become invisible. It will still be there, but we will not be able to see it.
The receding stars, galaxies and nebulae will seem to switch off and our sky will become a black void.
Perhaps there is a limit to the rate of expansion. Perhaps it will eventually reverse the process and begin to shrink. If so, we have plenty of time to develop advanced technologies. One must wonder that as agglomeration recommences, so life forms across the universe will seek ways to escape to another universe—to a parallel universe, perhaps—or to perish in the crushing heat of the current one.
Before closing let’s zoom in on our neck of the woods and see where all this leaves us. The nearest star is Proxima Centauri 4.28 light years away. Relatively speaking we are completely alone. We truly are in an insignificant corner, not particularly noticeable. Universes may expand, they may shrink, they may multiply. The timescales are beyond imagination. But we, here on Earth, may not have as much time as we think to observe what the universe gets up to billions of years from now.
In the early 1980s scientists noticed that mass extinctions on Earth seemed to occur every 27 million years. The culprit, according to a theory at the time is a brown star called Nemesis, residing about 1.5 light years away from the sun.
Such a star could affect the orbit of objects in the outer solar system, even as far out as the Oort Cloud, sending them whizzing into the inner solar system. Indeed, if Nemesis actually travelled through the Oort Cloud it could send billions of comets on a crash course towards Earth. Those favouring the Nemesis theory point out that most stars of the size and nature of our sun are binary: they have companion stars.
No one has seen Nemesis. If it exists it must be small and very far away. And if so, it would have an unstable orbit and be in no position to create a regular 27-million-year cycle of behaviour.
Scientists do agree on one thing, though, that there is strong evidence on Earth of major impact events every 27 million years. One interesting explanation concerns the orbit of the sun around the Milky Way, and that it oscillates every 27 million years—coincidentally. This would take the solar system through the denser central disk of the galaxy and expose our residency to a higher than normal exposure to planetary debris.
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By Nigel Benetton, science fiction author of Red Moon Burning and The Wild Sands of Rotar
Last updated: Saturday, 4th April 2020