Cosmology probes mysteries humans have wondered about since we first looked at the stars. Some answers are emerging.
Is space infinite?
Current models (supported by experimental evidence) assume the Universe is infinitely big and has been for the 13.7 billion years since it sprang into existence.
But language throws much confusion into the picture. We can only see part of the Universe ¯ only within a sphere, cantered at Earth with a radius the distance light travels in the 13.7 billion-year age of the Universe. The rest of the Universe is invisible to us.
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"So, I would say that stating the Universe is definitely infinite may be slightly overstating our claim, since we don't really know that it is infinite beyond our observable universe, but we have no strong reason to believe it is not infinite," astrophysicist Tamara Davis e-mails.
Also, the expanding nature of our Universe complicates understanding. A photon travelling for 13.7 billion years traverses more than 13.7 billion light years in distance, because the Universe is expanding. The photon moves through a space about three times larger (46 billion light years).
So, we should speak of two Universes ¯ the Entire Universe and the Visible Universe but often sloppily call them both by the same term: the Universe. That's the rub. So, when cosmologists speak of a "grapefruit" size early-time Universe, they mislead us, big time.
What cosmologists mean is the part of the Universe we now see (present Visible Universe) used to be grapefruit sized, astrophysicists Charley Lineweaver and Tamara Davis write in Scientific American. But the Entire Universe then (as now) could be infinite. "Shrink an infinite space by an arbitrary amount, and it is still infinite."
We must, however, exclude what happened at the time of the 'Big Bang', since we know nothing about that instant. 'Big Bang' time is when time began and our theories have validity only up to Planck time (10^43 seconds) after 'Big Bang' time.
Is there an actual centre of the Universe?
No there isn't a centre of the Entire Universe. Just a centre (at Earth) of our Visible Universe.
What's at the end of space?
The Universe doesn't end, as counter-intuitive as this seems. Once again, language confuses concepts. We speak of a 'Big Bang' but don't mean a 'bang' like an explosion, which has a centre and a shock wave that moves spherically out into air from the explosion.
Instead, the 'Big Bang' happened everywhere in the Universe at once, with no centre. Shortly after the 'Big Bang', density and pressure of the Entire Universe were the same everywhere. So, pressure difference could not possibly create an explosion. The 'Big Bang' wasn't a bang at all.
About 13.7 billion years ago, the Entire Universe increased by 10^30 (a million trillion trillion) times, in less than a second. We call this remarkable phenomenon, the 'Big Bang.' The Universe simply expanded ¯ not into anything because, even at the beginning, the Universe was probably infinite. (We can't say for sure the Universe was infinite at the exact beginning since our theories are valid only up to Planck time after the Big Bang). Thus, the Universe probably has no centre.
Sometimes analogies help understanding strange concepts like 'no end' and 'no edge.' Imagine we're a couple of ants living on a big red balloon. We think as two-dimensional creatures. We can't go up or down and move only on the balloon's surface. I, unfortunately, am stuck in a spot of honey and cannot move. But we wonder how big our world is. Where does it end? I'm fearful you'll fall off the edge but you insist on exploring the balloon. No matter which direction you set off, though, you always come back to me but can find no end to our world.
To complicate our life, but not enough to defeat us, a kid is blowing up the balloon as you wander. So, each circuit you make around our world takes longer than the one before (even though you crawl at the same ant speed) but you always come back to me and never encounter an edge. We conclude our world is expanding, distances are increasing, but our world has no end.
It's tempting to think we can increase the number of dimensions of our real-life space by one to explain why there is no end to the Universe. It works for the ants ¯ going from the 2-dimensional surface to the 3-dimensional balloon. The balloon is embedded in a 3-dimensional space and can expand into the 3-D space. We might conclude that the expansion of our space requires a fourth dimension for the Universe to expand into.
But the analogy breaks down for our 3-dimensional space and us. We do, indeed, live in a 4-D continuum. Time, however, is our fourth dimension ¯ different from the dimensions of space ¯ and the Universe changes with time. The fourth dimension helps express how the Universe changes, not what it expands into. We use the four dimensions (Space-time) to locate any physical object or event in the Universe ¯ where and when something is.
Einstein says space is dynamic. The Universe can expand, shrink and curve without being embedded in a higher dimensional space. Our space is self-contained, needing nothing else ¯ no edge, no end, and no nothing to expand into.
Is space constantly expanding?
You ask if the size of the Universe is defined by how far light has travelled since the Big Bang, and if the Universe is constantly expanding. The Visible Universe is defined by how far light has travelled since the Big Bang (see above).
The Entire Universe is continuously expanding but not at a constant rate. Expansion is accelerating.
As NASA reports, "...the results of the Wilkinson Microwave Anisotropy Probe (WMAP) mission and observations of distant supernova have suggested that the expansion of the universe is actually accelerating..." We knew the expansion is accelerating even before the early WMAP results were published," Lineweaver e-mails. In 1998, Lineweaver used cosmic microwave background measurements to verify experimentally the acceleration.
"The real clincher in the evidence for acceleration was observations of supernovae by the High-z Supernova Team and the Supernova Cosmology Project, who each published evidence for the acceleration of the universe, the first in 1998 by the High-z team, and then in 1999 by the SCP," Davis says. "Those observations are the ones that were considered decisive. They used supernovae as standard candles to measure distance, and combined that with the red shift to measure their recession velocity. From that they could tell that recession velocities have been increasing over time ¯ thus the acceleration."
An accelerating expansion implies the existence of a gravitationally repulsive form of energy. Cosmologists term this unknown form, 'dark energy.' "If dark energy in fact plays a significant role in the evolution of the universe, then in all likelihood the Universe will continue to expand forever," NASA says.
Unfortunately, we have little idea what dark energy is even though we estimate it comprises 74 per cent of the Universe's mass-energy.
Whatever causes our Universe to expand at an accelerating rate, the mere concept gives us deep insight. In 1929, astronomer Edwin Hubble formulated the expanding-universe idea into "Hubble's Law". The law states that that the rate at which the distance between us and a galaxy increases is directly proportional to its distance from us. So, a galaxy twice as distant from us recedes twice as fast as one only half so far away. About forty-five years ago, we verified Hubble's Law with experimental evidence.
Hubble was first to show that the Universe is expanding, and is considered by many to be the father of observational cosmology.
"Hubble law has been gathering increasingly precise observational validation ever since it was proposed," Lineweaver says. Furthermore, "the expansion and cooling [of the Universe]has been the unifying theme of cosmology, much as Darwinian evolution is the unifying theme of biology. Like Darwinian evolution, cosmic expansion provides the context within which simple structures form and develop over time into complex structures" ¯ the mind-boggling Universe we know today.
More exploring:
- Does cosmic expansion move everything apart, even the various parts of our bodies? WonderQuest, December 2008
- Misconceptions about the big bang , by astrophysicists Charles L. Lineweaver and Tamara M. Davis, Scientific American, March 2005
- The Visible Universe , Universe adventure, Lawrence Berkeley National Laboratory, August 2007
- The Cosmic Microwave Background and Observational Convergence in the Ωm-ΩΛ Plane, by Charles H. Lineweaver, The Astrophysical Journal letters, August 1998
- Experimental evidence supporting current cosmology models of the Universe, Wilkinson Microwave Anisotropy Probe (WMAP), NASA, April 2009
- Dark energy, dark matter, NASA, September 2009
- Ned Wright's Cosmology Tutorial, University of Los Angeles
(Answered 14 December 2009)
April Holladay lives in Albuquerque, New Mexico. Her column, WonderQuest, appears every second Monday of the month on WonderQuest.com. To read April's past columns, please visit her website. If you have a question for April, visit this information page.