Seeking SciNote, Physics: Is The Universe Infinite?
(image source: http://commons.wikimedia.org/wiki/File:Observable_Universe_with_Measurements_01.png)
Question: Is the universe infinite or finite?
asked by dispachio
Answer:
The universe is probably infinite—for at least one definition of “infinite.” Probably. Well, we don’t really know. In fact, we don’t even know for sure if it is possible to know. Here, however, are some things we do know:
Before we begin, we have to clarify what we mean by “the universe.” We can talk about the observable universe or the universe as a whole. The observable universe is a sphere centered on us within which the light from the beginning of the universe or afterward can reach us. Therefore, it is very finite and has a well-defined edge. The universe as a whole, on the other hand, extends far beyond that edge and is constantly expanding. Unfortunately, the universe had a definite beginning in time and nothing can travel faster than the speed of light; consequently, it is impossible for scientists to gain any data about what’s out there (although we can predict mathematically). The observable universe, therefore, extends only as far as the distance to those objects that would have emitted light 13.8 billion light-years away in the early stages of the universe. However, because the universe has expanded since then, those objects are now farther away from us, and so the observable universe is 92 billion light years in diameter. Like the universe as a whole, the observable universe is also constantly expanding as time passes and light from even more distant places has had time to reach us.
This brings up another point: whether the universe is infinite in space is one question, but it can also be infinite in time. Relativistic physics conceives of space-time as a unified framework in which matter and energy exist, so the universe extends, not just in spatial dimensions, but in temporal ones as well. It certainly doesn’t extend back infinitely far, as there was a definite starting point, but it could conceivably extend infinitely far into the future. The ultimate fate of the universe is unknown, but depends largely on its density and rate of expansion. For the future of the universe, two main possibilities exist (although there are other theories, such as the “Big Rip”): if the universe is highly dense and its expansion slows, eventually it will condense back together into a state like that of the Big Bang, an idea called the Big Crunch. Otherwise, if the universe isn’t dense enough to contract and its expansion remains constant or speeds up, eventually all matter will spread too far apart and will entropically decay into low-energy uniformity, a state called Heat Death.
(image credit: http://scienceblogs.com/startswithabang/2011/10/04/discover-the-fate-of-the-unive/)
The evidence available suggests that Heat Death is the most probable fate. The expansion of the universe is actually accelerating due to the presence of unknown dark energy, which accounts for the infamous Cosmological Constant that stymied Einstein and other physicists throughout the twentieth century. So, regardless of the size of the universe now, it’s constantly getting bigger, and will likely persist infinitely into the future, although not in the complex, high-energy state we’re used to. The expansion will eventually end as entropy becomes too great. Therefore, the universe can be considered to be temporally infinite; whether it’s spatially infinite, we can’t say.
Another facet we can examine is the shape of the universe, as we can measure the geometry of space-time from within the observable universe by looking at Baryon acoustic oscillations—disturbances in the cosmic patterns of matter—and fluctuations in the Cosmic Background Radiation—disturbance in the cosmic patterns of energy. These perturbations can tell us about the structure of the universe as it expands; and, therefore, can also tell us about its shape. The data we’ve found suggests that the universe is almost certainly very flat and this would imply that it extends on forever spatially, as well as temporally, because a plane is infinite. However, the only way we can theorize about the whole of the universe is to generalize from what we know about the observable universe and the laws of physics. These sorts of studies can help us predict what might lay beyond the observable universe, but also rely heavily on the assumption that what we can observe is a representative sample. If this assumption is not true, then the universe as a whole might defy many of our expectations. Just as the Earth seems flat while standing on it, it’s possible that the universe might seem flat from our own humble vantage point while having a curvature we can’t detect. It’s only recently that we’ve been able to gain the right theoretical framework and precision of technology to make measurements like this possible, and, when we’re talking about the scale of something beyond the observable universe, even tiny discrepancies can add up dramatically—especially since something can be bigger than we can imagine while still being finite. In the end, while we can say that the universe is probably infinite, the only thing we can be sure of is that the universe is far, far bigger than what we can see or know.
References and further reading:
What Is The Universe? and How Big Is The Universe? from Minute Physics
Dark Energy & The Big Rip and Cosmological Constant & The End of the Universe from Sixty Symbols
The NASA Wilkinson Microwave Anisotropy Probe project’s page on the shape of the universe
The Baryon Oscillation Spectroscopic Survey results confirm the universe’s likely flatness
If you’re really fascinated by this subject and want to go into more detail then, a few good books are also available:
Marcelo Gleiser’s The Island of Knowledge: The Limits of Science and the Search for Meaning talks about why we might never be able to know if the universe is infinite.
John Barrow’s The Infinite Book: A Short Guide to the Boundless, Timeless and Endless is a good introduction to ideas of infinity in mathematics and physics.
Answered by Lauren W., Expert Leader, and Brendan C., Expert
Edited by Emily S.
If you have any science-related questions for our team of Experts at SciNote, please drop by our ask box to submit your questions!
jbbartram-illu
gummygunk
snowghoul
janainaart
izzy806
pixelskylines
kianamaiart
blakekathryn
