... and continues to expand
Ripples flow through space and matter clumps together – ordinary attractive gravity [1] causes galaxies and other heavenly bodies to form
As the universe cooled and expanded at a colossal rate, doubling in size every 10-37 seconds, it did not do so evenly. Instead it clumped together forming stars, galaxies, planets (and eventually human beings), leaving astronomers to wonder why the infant universe did not simply spread out evenly after the big bang creating a cosmos filled with nothing more than a uniform mist of matter. It has recently been discovered, with the help of the Anglo-Australian Telescope at Siding Spring, near Coonabarabran, that this clumping together of matter was caused by “quantum jitters” or ripples flowing through space shortly after the big bang, thereby destabilising the initial uniform “mist” of matter[2]. They now became the seeds around which clumps of matter grew. Regions slightly denser than their surrounds pulled in more matter, making them denser still, increasing their gravity still more.
In other words, gravity commenced making structure. Remember that gravity had separated from the other forces at the Planck time, 10-43 seconds after the big bang, and the gravitational pull of regions of different densities rendered the universe less and less uniform, resulting in the dense clumps of matter of different masses and densities we see about us now. The wiggles in space-time caused dark matter to “fall in” to these dimples and form diffuse haloes. Since the dark matter particles were only weakly interacting, these haloes stayed diffuse. However, ordinary matter was also pulled in, forming dense clouds of gas. Eventually, this matter developed into a web of filaments with voids separating the denser regions. We see these filaments and voids today in the distribution of galaxies[3].
This explanation is confirmed by photographic evidence of miniscule variations in temperature in the cosmic background radiation shortly after the big bang, reflecting tiny variations in the distribution of matter. Further, in 2002, astronomers in an observatory high in the Chilean Andes, so high in fact that they had to carry oxygen bottles, were able to take photographs of this radiation as it was some 300,000 years after the bang. It revealed even at that stage the miniscule clumps of matter from which all the planets, stars and galaxies evolved.
The new measurements also supported the concept of a rapid, violent explosion a split second after the bang. Differences in the temperature of the background radiation as small as 10 millionths of a degree were detected, reflecting tiny variations in the distribution of matter. Had the background radiation been completely smooth, there would have been no explanation for why matter coalesced into bigger lumps[4].
Above: A schematic outline of cosmic history viewed vertically.
Source: indicated on image. Subsidiary source: CCE course Origins: From the Big Bang to Life, March 2011
The first stars are born - and some new evidence [4.1]
Having lasted for over 200,000 years, the Cosmic Dark Age came to an end. The densest regions contracted and heated up under the influence of gravity so much so that hydrogen could fuse to helium. Some 180 million years after the bang, the first stars in the universe were born, and in the meantime, the universe continued to expand.
And then something truly remarkable happened. the ultraviolet radiation from these first stars changed the electron spin in the hydrogen atoms, causing it to absorb the background radio emission of the universe at a natural resonant frequency of 1,420 MHz and casting a shadow, so to speak. In 2015-2016, scientists detected these background emissions at a much lower frequency (78 Mhz) some 13.8 billion years later, the universe having expanded nearly 18-fold in that time. Expressed in plain language, what has been detected is the absorption of light by the neutral atomic hydrogen gas, which filled the early universe after it cooled down from the hot plasma of the Big Bang.
Operating in the band of 50-100 Mhz (which incidentally overlaps some well-known FM radio stations), the discovery was made by a small radio antenna known as The EDGES ground-based radio spectrometer in a remote part of the Western Australian desert This is said to be the most important astronomical discovery since the detection of gravitational waves in 2015, because the first stars represent the start of everything complex in the universe: the beginning of the long journey to galaxies, solar systems, planets, life and brains.
The signal is about twice as strong as that expected and tells us the hydrogen gas at this time is significantly colder than anticipated according to the standard model of cosmic evolution. It is speculated that this may be because the hydrogen atoms may be interacting with cold dark matter. Cold dark matter was introduced as a concept in the 1980s to explain how galaxies rotate – they seemed to spin much faster than can be explained by the visible stars and an extra gravitational force was needed.
It is now thought that dark matter has to be made of a new kind of fundamental particle. There is about six times more dark matter than ordinary matter and if it was made of normal atoms the Big Bang would have looked quite different than it presently appears. “So if cold dark matter is indeed colliding with hydrogen atoms in the early universe and cooling them, this is a major advance and could lead us to pin down its true nature. This would be the first time dark matter has demonstrated any interaction other than gravity”.
The findings await further verification once the $1 billion Square Kilometre Array, the largest and most-expensive radio telescope in the world is up and running post 2020 in the same remote patch of the western Australian desert, and in the meantime a plea “Let’s hope the governments of the world, or at least Australia, can keep the frequency of 78 MHz clean of pop music and talk shows so we can continue to observe the birth of the universe”
Reionisation
The first stars were quite different to stars in the universe today. Because the only elements were hydrogen and helium, they contained none of the heavier elements present in all stars today. Models show that such stars would be much brighter and much more massive than the heaviest stars today. The first stars did more than just light up the universe: they re-ionised it causing it to radiate energy[5]: that is, the energetic photons of ultraviolet light stripped the electrons off the hydrogen atoms, allowing the blue light to travel freely. The birth of the first stars as they lit up in fusion is the second threshold considered on the Big History site. In the process they endowed the Universe with a new complexity critical to the formation of galaxies, larger clusters, and superclusters.[6]
The death of the first stars had major consequences in two ways. When they went supernova (violently exploded), they scattered heavy elements throughout their surroundings, which then became incorporated into subsequent generations of stars, and ultimately our planet and us as human beings. In addition, the collapsing cores of these stars probably left behind black holes, which may have provided the seeds which grew into the massive black holes we see at the centres of quasars and galaxies today.
What caused reonisation? [7]
What actually caused reonisation is still a matter of some debate in astronomical circles. Was it the first stars - gaseous giants composed of hydrogen with a little helium and lithium – which continued to grow to a size of anything from 100 to 100 million suns’ worth, until they got hot enough to initiate fusion and explode? Or was it the gaseous belches from black holes that originated from the collapse of these giants, swallowing gas voraciously until they became so hot that some of this gas spewed back into space in the form of jets, known as quasars, shining so brightly that the light can be seen half way across the cosmos, or gamma-rays, which slam into surrounding clouds of gas, triggering a secondary, bright afterglow of visible and infrared light, capable of being seen by conventional telescopes?
Or perhaps, later in the piece, it was the process of formation of the first galaxies which should be given the credit. On our present range of vision, there do not appear to be enough of these galaxies to produce sufficient ultraviolet radiation to reonise all the neutral hydrogen. From our present vantage point, a significant fraction of ultraviolet light appears to be missing from many of them, having been absorbed by the surrounding neutral hydrogen, until it was all absorbed about a million years post big bang when the universe became fully reonised and the cosmos fully transparent. It may be a reasonable assumption that there are galaxies at the edge of our present field of vision which are simply too dim to see with any existing telescope. If this is the case, it may mean that galaxy formation was the catalyst for reonisation. Certainly, the required energy cannot have come from black holes, given how difficult it is to make enough supermassive black holes quickly enough so to do, whereas the process of galaxy formation appears to have been going on for at least 100 million years already.
A recent experiment (2018) called the Reionization Lensing Cluster Survey (RELICS) aimed to find some of the first galaxies to form in cosmic history, using the technique of gravitational lensing [8]. The technique yielded more than 300 ancient galaxies, including one named SPT0615-JD whose light began its journey toward Earth 13.3 billion years ago, in other words, some 300,000 years after the universe began. The technique thus provides a window into cosmic history. It is conjectured that galaxies such as SPT0615-JD transformed early space by blasting out ultraviolet light that the gas around them absorbed, turning the universe's first neutral atoms back into the lone protons and electrons that they started out as, reionisation in other words. However, the details of how and when this process occurred are still unclear [9].
Apart from alluring alternatives such as these, the answer lies beyond our reach at the moment but computer simulations of the early universe and technological advances such as those provided by the James Webb space telescope (awaiting launch in 2021) with the capacity to peer right back to and perhaps beyond the Big Bang threshold, may yet provide us with the answers.
[1] It is necessary to distinguish between gravity considered as a force and gravity under general relativity. Under general relativity, gravity is not a force at all if that concept is considered as something that exercises a pull or a tug upon another object. General relativity gravitational theory is postulated on the basis that objects moving in space travel along the shortest possible paths called geodesics, which dictate the movement of freely-falling matter through space-time. Gravity and the curvature of space-time do not make a body depart from a geodesic, so neither of them is strictly speaking a force. This is the subject of later elaboration.
[2] ‘Universe’s big secret – where there’s a wiggle, there’s a way’, SMH 13, Jan 2005.
[3] CCE Course: Origins; Greene (2005), Ch 11. The role of dark matter in the clumping together of ordinary matter also featured in the exhibition From Earth to the Universe at the Power House Museum Exhibition, Sydney, op.cit.
[4] ‘When the universe was just a bub, it looked like this’, SMH, 25-26 May 2002.
[4.1] This segment is an edited version of "A signal from the First stars" by EarthSky Voices in Space | March 3, 2018 , article by Karl Glazebrook from Swinburne University of Technology, at earthsky.org/space/signal-from-1st-stars-detected
[5] Ion: an atom or molecule in which the total number of electrons is not equal to the total number of protons, giving it a net positive or negative electrical charge.
[6] https://www.bighistoryproject.com/chapters/1#star-formation
[7] Source (unless otherwise stated): Michael D. Lemonick, “The first starlight”, Scientific American, April 2014, 24-31.
[8] What follows is an edited summary of portion of Dan Coe's "Back in time" article which in the Scientific American, November 2018, 34 ff.
[9] Ibid, 36, 38.