Cosmologists have discovered a great deal about the evolution of the universe over its 14 billion-year history and have traced the origins of the universe back to a mere split second after "time zero," when the universe was filled with a primordial "soup" of extremely hot, dense matter and radiation. Light is an example of radiation, and being in this soup would have been like being at the centre of the Sun, except exceedingly more intense.
Despite this tremendous progress, many profound mysteries remain, the deepest being: how did the universe actually come into being – possibly out of nothing – during this split second, called the Big Bang phase?
The Essence of Cosmic Evolution
Astronomical observations indicate that the size of the universe – space itself – has always been expanding, and that matter and radiation contained in that space have always filled it more or less uniformly. As space expands, matter and radiation are spread more thinly. They become less dense and cool down, allowing more fundamental particles to "condense" into larger, composite particles – in much the same way water vapour condenses into droplets as it cools. This is the essence of cosmic evolution.
By the time the universe was about three minutes old, the primordial soup of subatomic particles had cooled enough to allow simple atomic nuclei, primarily hydrogen and helium, to condense out. A few hundred thousand years later, the universe had cooled still further, enough for these atomic nuclei to capture electrons, allowing even larger particles – atoms – to condense out.
Prior to this time, a photon (a particle of light) couldn’t travel very far before colliding with an electron or an atomic nucleus, making the universe opaque, like a dense fog. After this time, photons could travel largely undisturbed, and the universe became transparent, as it is today.
Most photons born at that time still exist, having traveled through empty space for some 14 billion years, without ever colliding with matter! They began as "hot" photons (about 3000 K, which would appear deep red to the human eye), but due to the expansion of the universe have now cooled into microwave photons like those used in satellite television transmissions (about 2.7 K, close to absolute zero temperature). They fill all of space (think of a "photon gas"), and carry with them detailed information about what the universe was like at the time they were born. Photons come at the Earth from all directions, and in recent years, satellites such as the Wilkinson Microwave Anisotropy Probe (WMAP) have taken increasingly precise snapshots of this so-called cosmic microwave background (CMB) radiation, as in the image below.
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In this image, the colours represent the various temperatures of the CMB photons coming from different directions in the sky: dark blue indicating slightly lower than the average temperature (of 2.725 K), and red, slightly higher, with gradations in between. Considering that each CMB photon has been travelling in a straight line, at the speed of light, for nearly the age of the universe, it must have been born about 14-billion years ago in a region of space about 14 billion light years away in the direction from which it came. This image is thus a snapshot of a distant spherical shell of the universe as it looked a few hundred thousand years after time zero. It is a remarkable image representing the furthest it is possible to see from our vantage point, here and now, on the Earth. Space almost certainly continues well beyond this – how far is impossible to know. Just like looking out to sea, here on the Earth, this is our "cosmic horizon," beyond which we cannot see.
What does this image tell us? First, since all CMB photons have almost exactly the same temperature (2.725 K), regardless which direction they come from, matter and radiation in the early universe must have been almost perfectly uniform in temperature, and thus density. Second, it's only almost perfectly uniform. The tiny temperature variations in the WMAP image (only about 400 micro-Kelvin between hottest and coldest), depending on the direction we look, translate into tiny variations in the density of matter and radiation at different places – on the spherical shell, and presumably throughout all of space – in the early universe. It is believed that these slight density variations were the seeds of gravitational collapse that, over cosmic time, grew into the stars, galaxies, clusters of galaxies, super-clusters, and so on, that we see today.
Where did these density variations come from? Imagine a microscopic region of space filled with our primordial soup, a fraction of a second after time zero. The matter and radiation would have been teeming with subatomic-scale quantum fluctuations, much like the "noise" on your television screen between channels. It is believed that in much less than the blink of an eye, all such microscopic regions of space simultaneously expanded to cosmic size, "freezing" their quantum fluctuations in place, and magnifying them to the cosmic-scale temperature variations seen in the WMAP image. In other words, the density variations implied by this image are believed to be actually a hugely magnified view of the quantum fluctuations in an ultra-microscopic region of the universe much earlier in time – a tiny fraction of a second after time zero. What an incredible image!
The Dark Side of the Cosmos
Much of the picture of cosmic evolution sketched above, called standard cosmology, is well grounded in fundamental physics, but makes up only part of the story. Standard cosmology involves detailed models, whose predictions agree with, and explain, much of what astronomers see. However, there are a growing number of observations that are deeply puzzling.
For example, a number of independent astronomical observations have provided strong evidence for the existence of vast quantities of matter that do not emit or reflect electromagnetic radiation of any type (visible light, microwaves, gamma rays, etc.), and thus cannot be seen. It is called dark matter. How do we know it's there? Even though we cannot see it, it exerts very clear gravitational influences on the matter and radiation we can see.
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For example, Albert Einstein's theory of space, time, and gravity, called general relativity, tells us that any gravitating mass (the Sun, a galaxy, a cluster of galaxies, etc.) warps the spacetime around it in such as way that a light ray passing nearby is deflected. Gravity bends light. Astronomers find that the amount of bending around, say, a typical cluster of galaxies, is far greater than can be accounted for by the visible mass in the cluster. There appears to be a great deal of invisible mass. The image below shows a typical 3-dimensional map of the distribution of dark matter obtained by such light-bending observations. Current data suggests that there is more than five times as much dark matter as ordinary matter (atoms) in the universe. What is dark matter made of, and can it be detected in laboratories here on Earth? An intense, worldwide effort is currently underway to try to answer these questions.
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Theoretical reconstruction of the distribution of dark matter in one part of the universe, based on observations by the Hubble Space Telescope [more]. Credit: NASA, ESA, and R. Massey (California Institute of Technology).
Another profound puzzle stems from astronomical observations indicating that the cosmic expansion of space is happening at an accelerating pace. But in a universe with only matter (dark or otherwise), gravitational attraction would slow down the expansion, just like a ball, thrown upwards, slows down due to Earth's gravitational pull. The acceleration can be explained by the assumption that the universe is filled with an unusual form of energy called dark energy that makes up 70% of the universe's total energy. But what, exactly, is this dark energy, and how does it fit in with the rest of physics? To date, no one knows the answer.
Back to the Bang
As we trace cosmic history closer and closer to time zero, our knowledge of fundamental physics becomes less and less reliable. Eventually, temperatures (and thus particle energies) and densities become so extreme that we leave the domain of all well-tested physical theories. Even so, we can devise models for these early stages in the evolution of the universe. The best known such model is inflation, which describes the extremely rapid expansion mentioned above in our discussion of the WMAP image. There is some evidence for this kind of inflationary phase, but the underlying fundamental physics is not yet understood.
Travelling back even further, we encounter energies and densities that will remain beyond comprehension until we achieve a detailed understanding of the quantum nature of space, time, and gravity itself: a theory of quantum gravity. In much the same way that quantum theory is required to understand atoms, without a theory of quantum gravity, we cannot hope to answer fundamental questions about the beginning of the cosmos. While only an incredibly tiny fraction of a second in duration (about 10-43=0.0000000000000000000000000000000000000000001 seconds), this so-called Planck epoch of the Big Bang contains some of the deepest and most profound mysteries ever contemplated.
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Evolution of the universe from quantum fluctuations, to inflation [more]. Credit: NASA/WMAP Science Team.
To learn more about cosmology at Perimeter Institute and the researchers, please click here.
Perimeter Institute Resources
The Mystery of Dark Matter (basic)
This video about dark matter can be enjoyed online and is also available as a kit including a Teacher’s Guide for in-class use by educators.
The following selection of Perimeter Institute multi-media presentations by leading scientists is particularly relevant to cosmology. Click on the link to read a full description of each talk and choose your viewing format.
- Einstein – Relativity and Beyond – John Moffat, Lee Smolin, John Stachel, Howard Burton (moderator) (basic)
- The Big Bang – Simon Singh (basic)
- The Quantum and the Cosmos – Edward (Rocky) Kolb (basic)
- Strange Views of Space and Time – Gary Horowitz (basic to intermediate)
- Faster than the Speed of Light: Could the Laws of Physics Change? – Joao Magueijo (intermediate)
- Fundamental Physics in 2010 – Nima Arkani-Hamed (intermediate)
- What Banged? – Neil Turok (intermediate)
- Einstein’s Vision and the Quantum Universe – James Hartle (intermediate to advanced)
- The Origin of the Universe – Steven Weinberg (intermediate to advanced)
These multi-media talks by Perimeter Institute researchers and visiting scientists were presented to youth and educators during Perimeter Institute’s ISSYP, EinsteinPlus or other occasions.
- Cosmology – Robert Brandenberger (basic)
- Dark Matter – Mark Ackersviller, Michael Burns (basic)
- Dark Matter – Lee Smolin (basic)
- Resolving the Mystery of Dark Matter – James Taylor (basic)
- What’s blowing up the Universe? – Mark Wyman (basic)
Suggested External Resources
- WMAP: Universe 101 – Big Bang Theory - A concise introduction to the Big Bang and the cosmic microwave background radiation
- WorldWide Telescope - Downloadable software to view NASA images (like WMAP) as in a planetarium
- Einstein Online Spotlights: Cosmology - Articles about various aspects of cosmology (basic)
- Ned Wright's Cosmology Tutorial - A step-by-step introduction to the cosmological standard model (intermediate)
- SNOLAB - Some of Canada’s contributions to the experimental search for dark matter
- Hogan , Craig J. The Little Book of the Big Bang: A Cosmic Primer. Springer Publishing, 1999. (basic)
- Weinberg, Steven. The First Three Minutes: A Modern View of the Origin of the Universe. Basic Books, 1993. (intermediate)