The Universe and What Happened When it All Began
Many of us have wondered at some point about the origin of the Universe: Where did it come from? How will it develop in the future? Does it actually have a start or an end? These questions, fundamentally linked to our very own existence, have sparked interest and excitement of generations of scientists and laymen as well as many science fiction authors and filmmakers.
Three weeks ago a group of scientists (called the BICEP2 collaboration) announced the results of their investigations using a telescope at the South pole targeting traces of the birth of the Universe. Many newspapers coined catchy headlines when reporting the BICEP2 results. The Guardian started it all off with “Gravitational wave: have US scientists heard echoes of the big bang?” and later added “Primordial gravitational wave discovery heralds ‘whole new era’ in physics”. The BBC news website went with “Cosmic inflation: ‘Spectacular’ discovery hailed” and New Scientist published two articles, one headed “First glimpse of big bang ripples from Universe’s birth” and the second titled: “Multiverse gets real with glimpse of big bang ripples”.
A brief history of cosmology
To understand the meaning of all of these headlines and in order to be able to judge whether they are a good representation of what the BICEP2 scientists actually announced or if they might dip deeper into the speculative spectrum, we have to recap what cosmologists have discovered during the past 100 years. We need to understand what is referred to by the cryptic terms of ‘cosmic microwave background’, ‘inflation’ and ‘gravitational waves’.
In the 1920’s Edwin Hubble observed that most of the galaxies around the Milky Way, our home galaxy, are drifting away from us. He was even able to quantify their retreat and he found that the further a galaxy is away from us the faster it actually retreats from us. His observation, referred to as Hubble’s Law, turned our view of the Universe upside down, because first of all it meant the Universe is not static. Secondly, if nowadays we see everything drifting apart in all directions and expanding, it means that at some point in the past even the remotest parts of the Universe must have been very close together. So, we can already answer one of the questions we started with, i.e. we know that the Universe had a beginning. If we trace the movement of the galaxies back in time we can actually determine that about 14 billion years ago the whole Universe must have been compressed into an infinitesimal small volume. At that point the universe burst into existence, what astronomers now refer to as the Big Bang, and began expanding rapidly.
Can we still detect echoes of the Big Bang?
The immense density of all the matter in the Universe compressed into a tiny volume is impossible to picture, but we can easily imagine what a powerful and energetic event the Big Bang actually was. Everything was so hot and energetic that atomic nuclei could not exist and only came into existence a few seconds after the Big Bang. Even light could not escape from this hot, dense phase and it was only after 380,000 years of existence that the Universe had expanded and cooled down enough to become transparent to visible light. Due to the continuous expansion of the Universe this visible light must have shifted in frequency over the past 13.5 billion years. In the 1960s Arno Penzias and Robert Wilson stumbled upon this shifted light – the so-called cosmic microwave background (CMB) – and both were awarded the 1978 Nobel Prize in Physics for their discovery.
The discovery of the cosmic microwave background and the subsequent analysis of its properties have been in the focus of cosmology ever since. Why is that? As it turns out the CMB is surprisingly homogenous and varies by less than 0.1% across the whole Universe. The majority of cosmologists today believe that this can only be explained by extremely rapid expansion of the Universe in the first 10-31 (less than millionth of a millionth of a millionth of a millionth of a millionth) seconds of its existence. It is believed that the Universe increased in size by a factor of 1026 (or a hundred million million million million) in a fraction of a second. For me as a physicist, inflation is one of the most weird and mindboggling concepts. However, inflation represents to date the best explanation of what happened right after the Big Bang, why we see such a homogeneous CMB and why stars and galaxies could form in the Universe. Just before inflation started the Universe was so small that it could be in a thermal equilibrium, i.e. any fluctuations in temperature could balance each other out over the whole mini-Universe. That is why we nowadays measure such an amazingly homogenous CMB across the whole Universe. However, at this pre-inflationary stage of the Universe, quantum fluctuations must have been present, which caused tiny differences in temperature and density. During the inflation process, these tiny quantum mechanical fluctuations propagated into macroscopic features and have been imprinted on one hand as the tiny variations in the CMB, but on the other hand also as large-scale density fluctuations serving as seeds for star and galaxy formation, giving the Universe its current structure.
What are gravitational waves?
Now, we are closer to understanding the BICEP2 results. The last remaining concept we have to understand is what a ‘gravitational wave’ is. The existence of gravitational waves is a direct consequence of Albert Einstein’s theory of general relativity. He predicted that if compact but heavy masses, such as certain types of stars, undergo extreme acceleration, then this will cause small ripples in spacetime, the fabric of the Universe, similar to waves on the surface of a pond when a stone is thrown in. Though many scientists across the globe have been working on measuring these gravitational waves for decades, they are so tiny that nobody has directly detected them so far. However, we have good reason to believe that gravitational waves are real: in 1993 Russell Hulse and Joseph Taylor were awarded the Nobel Prize for Physics for observing a double star system and quantifying the a change of the orbital period, i.e. the time it takes for the two stars to circle around each other. They found the two stars speeding up in their circular dance, exactly in the same way as it would be expected according to Einstein’s theory of General Relativity due to the double star system losing energy by radiating gravitational waves.
So, most scientists believe in the existence of gravitational waves. However, what happens now if we combine the existence of inflation with the existence of gravitational waves? This is a question that has been investigated by several theorists and cosmologists over the past few decades. Obviously, if inflation has taken place then huge masses, i.e. the whole Universe, with its imprinted density fluctuations, must have been accelerated rapidly and therefore caused strong, primordial gravitational waves. Certain theories now predict that these gravitational waves have been imprinted into the properties of the CMB, as so-called “B-mode polarisation”. Polarisation is a property of any electromagnetic wave, which describes the direction in which the electric field oscillates. For example the light reflected from a water surface or a glass window has a polarisation parallel to the reflecting surface. According to the above-mentioned theory, the CMB radiation reflected off density fluctuations in the inflated Universe would have created a specific polarisation pattern, and these B-modes should still be visible in the CMB even today.
What did the BICEP2 scientist actually discover?
The telescope at South pole run by the BICEP2 collaboration was especially designed to search for the B-mode polarisation signature in the CMB. After three years of observation (2010-2012) and two further years of analysis, the BICEP2 scientists are convinced they really have found statistically convincing evidence for the existence of B-mode polarisation of the CMB. They published their results in two articles posted at the arxiv preprint server (so these articles are not yet peer-reviewed). In contrast to many of the corresponding news headlines, the BICEP2 scientists used rather demure and factual titles for their two articles: “BICEP2 I: Detection of B-mode Polarization at degree angular scales” and “BICEP2 II: Experiment and three-year data set.” which reflect very precisely what these articles are about. I personally believe the BICEP2 collaboration needs to be congratulated not only on their amazing discovery, but also on the style of their two articles, which are very thorough and nearly completely avoid speculation.
How accurate have the headlines been on the BICEP2 news coverage?
The next few months and years will show if other CMB observation experiments, currently in operation on the ground and in space, can independently confirm the discovery of the B-mode polarisation of the CMB. If the BICEP2 results were confirmed, it would be quite sensational! In that case the conclusions would be that we have strong evidence in our hands that 1) inflation or a similar bizarre process has really taken place a fraction of a second after the Big Bang and 2) that primordial gravitational waves exist and have imprinted their signature into the CMB. So let us go back to the headlines and check their appropriateness. Guardian’s headline “Gravitational wave: have US scientists heard echoes of the big bang?” is spot on in terms of contents and formulating it in a question nicely reflects, that peer-review of the articles and confirmation by independent experiments are pending. “Primordial gravitational wave discovery heralds ‘whole new era’ in physics”, “Cosmic inflation: ‘Spectacular’ discovery hailed” and “First glimpse of big bang ripples from universe’s birth” are solid and capture the essence of the BICEP2 results. Only the heading of the second New Scientist article “Multiverse gets real with glimpse of big bang ripples” seems to go beyond what the BICEP2 collaboration really published and starts to speculate about consequences further afield.
So what do the BICEP2 results mean for me, as one of the numerous scientists who are hunting with instruments like GEO600 or Advanced LIGO for the first direct detection of gravitational waves? It is really exciting news and good to see that in addition to the Taylor and Hulse results, there is more indirect evidence for the existence of gravitational waves. However, I am rather doubtful that BICEP2 results can be seen as the first direct detection of gravitational waves. What BICEP2 claims to have measured by observing radio waves is ‘only’ the effect of gravitational waves from nearly 14 billion years ago on the CMB. So our hunt for detecting gravitational waves impinging on the earth today and disturbing the spacetime around us continues.
Is the Universe weird?
On a final personal note: I have to admit that I was never a big fan of any cosmic inflation theories. Somehow it is rather unsatisfactory as a scientist to accept such a bizarre concept, which is so counterintuitive, and which disagrees with the natural laws we usually take for granted. It is hard to explain what was the driving factor for inflation, why it started at all and even more weird, why it suddenly stopped after a tiny fraction of a second. However, if the BICEP2 results are confirmed, then I have to accept inflation and consent to the fact that our Universe is weirder than I would like it to be. Perhaps the following quote from Douglas Adams, from his novel “The Restaurant at the End of the Universe” can give us some comfort:
“There is a theory which states that if ever anybody discovers exactly what the Universe is for and why it is here, it will instantly disappear and be replaced by something even more bizarre and inexplicable. There is another theory which states that this has already happened.”
“BICEP2 I: Detection Of B-mode Polarization at Degree Angular Scales”, BICEP2 Collaboration (P.A.R. Ade et al.), Mar 16, 2014. arXiv:1403.3985 [astro-ph.CO]
“BICEP2 II: Experiment and Three-Year Data Set”, BICEP2 Collaboration (P.A.R. Ade et al.), Mar 17, 2014. arXiv:1403.4302v2 [astro-ph.CO]
Image shows a full-sky temperature map taken by NASA’s Wilkinson Microwave Anisotropy Probe at 94 GHz: 2003 version. Temperature range of ±200 mK is shown. Attribution: By Chrisbrl88 at en.wikipedia Later version(s) were uploaded by Khailarkin, Papa November at en.wikipedia. [Public domain], from Wikimedia Commons