“Space is big,” said Douglas Adams. “You just won’t believe how vastly, hugely, mind-boggingly big it is.” But why must this be so? And why does our Universe exhibit such tremendous scale, from the very tiny to the extremely large? Here are some possible answers.
From Micro to Macro
It’s hard to wrap our heads around the size of our galaxy, let alone the Universe. At 100,000 light-years across, it would take the New Horizons space probe—the fastest object ever launched by humans—some 1,844,000,000 years to travel from one side of the Milky Way to the other (it’s currently moving away from Pluto at 58,536 km/h or 36,373 mph).
But there are structures even larger than single galaxies. Back in 2013, astronomers discovered a concentration of galaxy clusters stretching some 10-billion light-years across. There are also cosmic filaments to consider—massive strands of rarefied and highly ionized gas which stretch like spider webs across the observable Universe linking galaxy clusters across billions upon billions of light-years. And then there’s the Universe as a whole, an expanse of expanding space that appears to be about 92 billion light-years in diameter. And that’s just the observable Universe.
Universe Today: How Big is the Universe?
But when it comes to size, there’s more than the universe’s cosmic vastness to consider. There’s also its tremendous scale. Heading in the opposite direction, the microscopic world is similarly vast in scope. There are roughly 9.6 x 1024 molecules in a single glass of water, while the human body is comprised of about 7 x 1027 atoms, where the size of a single atom is about 10-8 centimeters. But once we get down to the Planck length, roughly 10-33 centimeters, we enter into the realm of the absurdly small. Beyond this scale no meaningful measurements can be made, and classical physics breaks down.
From our perspective as humans, this vast range, from the smallest to the largest, is beyond extreme. It’s inconceivable. But why should this be? Why can’t the Universe be packed down into a more “manageable” size? Why the degree of micro, and why such macro? To us, this seems like a tremendous waste of space.
Anthropic Bias
One person who has given this subject considerable attention is theoretical cosmologist Sean Carroll, a research professor in the Department of Physics at the California Institute of Technology. As he explained to me, these are certainly interesting questions—but they’re not necessarily the right questions to be asking.
(Credit: O Chul Kwon)
“Whenever we seem surprised or confused about some aspect of the universe, it’s because we have some pre-existing expectation for what it ‘should’ be like, or what a ‘natural’ universe might be,” Carroll told io9. “But the universe doesn’t have a purpose, and there’s nothing more natural than Nature itself—so what we’re really trying to do is figure out what our expectations should be.”
Carroll says it’s not surprising that humans are small compared to the universe, but big compared to atoms. That’s a feature of our existence with an obvious anthropic explanation.
“Complex structures can only form on in-between scales, not at the very largest or very smallest sizes,” he says. “Given that living organisms are going to be complex, it’s no surprise that we find ourselves at an in-between size compared to the universe and compared to elementary particles.”
Which is both a good and obvious point. The Universe appears the way it does to to us on account of our position as biological organisms. We’re evolutionarily biased to perceive the Universe the way we do, both as creatures that formed in the “in-between scales,” and owing to our psychological and cognitive predispositions, including our inability to comprehend large numbers and sizes beyond a certain point.
Chandra Observatory astrophysicist Jonathan McDowell, who works out of the Harvard-Smithsonian Center for Astrophysics, extends this line of thinking even further by invoking the anthropic principle, which states that observations of the Universe must be both consistent and compatible with the conscious life that observes it.
“It is the vast numbers of elementary particles and of space-time geometry quanta that give the illusion of continuity and the emergence of complexity,” McDowell told io9. “We will eventually discover that our laws of physics are natural byproducts of the behaviour of rearrangements of 10-to-the-very-large-power particles.”
In other words, the Universe has to be this way, otherwise we wouldn’t be here to observe it. Moreover, our “perception” of the Universe is hideously limited, and even illusory, as McDowell points out.
Crafted by the Constants
And further to McDowell’s point, we must also consider the laws of the Universe. According to Brian Koberline, an astrophysicist at the Rochester Institute of Technology, the apparent size of the Universe is a function of the strength of different physical constants.
“If they were different the ‘scale’ of the universe would be different,” he told io9.
A hydrogen atom’s orbital structure (credit: APS/Alan Stonebraker)
As an example, Koberlein points to the strength of the electromagnetic field—a physical law that has determined how large creatures like us can be; if it were weaker, atoms would be larger. Or, had the universe sprung into existence with a stronger gravity, it would have collapsed back into a dense state a long time ago.
Koberlein says that the speed of light is also a factor—a built-in speed limit that will preclude us from perceiving the Universe as anything but an inconceivably massive expanse of space.
“It limits how far we can travel in our lifetime, and even whether we can reach some of the most distant galaxies—which we can’t,” he says. “All of these physical constants determine the scale of a universe like ours.”
The “Preposterous” Universe
There’s also something very interesting—and even bizarre—about the size of the Universe as it pertains to particle-physics scales. The Planck scale is much, much smaller than the level of the atom. According to Sean Carroll, the difference between these two numbers is puzzling—an issue that’s related to the “hierarchy problem” of particle physics. At the same time, however, the scale of the universe is roughly 1029 centimeters across, which is enormous by any scale of microphysics. Carroll says it’s perfectly reasonable to ask why.
(Credit: NASA/CSC/M. Weiss)
“Part of the answer is that ‘typical’ configurations of stuff, given the laws of physics as we know them, tend to be very close to empty space,” says Carroll. “That’s a feature of general relativity, which says that space is dynamical, and can expand and contract. So you give me any particular configuration of matter in space, and I can find a lot more configurations where the same collection of matter is spread out over a much larger volume of space. So if we were to ‘pick a random collection of stuff’ obeying the laws of physics, it would be mostly empty space. Which our universe is, kind of.”
But as Carroll points out, even empty space has a natural length scale, which is set by the cosmological constant, which is the value of the energy density of the vacuum of space. Back in 1988, physicists discovered that the cosmological constant is not quite zero, although it’s very small. The length scale that it establishes—which is roughly the distance over which the curvature of space becomes appreciable owing to the cosmological constant—just happens to be the size of the universe we see today. Interestingly, because the cosmological constant is inversely proportional to this length scale, Carroll says that the question, “Why is the cosmological-constant length scale so large?” is the same as, “Why is the cosmological constant so small?”
All this raises two rather important questions. First, the Universe is expanding, but the length scale associated with the cosmological constant is a constant—so why are they roughly equal today? This is what cosmologists refer to as the “coincidence problem.” Secondly, why is the cosmological constant scale so enormously larger than the Planck scale, or even the atomic scale for that matter? Carroll says there are no widely-accepted answers to either of these questions.
The Universe is so big, says Carroll, because the cosmological constant is so small. As to why the cosmological constant is so small, that remains a scientific mystery.
Note: An earlier version of this article incorrectly described “a smaller gravity” to collapse an early version of the universe. It should have read stronger.
Note 2: An earlier version of this article incorrectly noted the scale of the universe at roughly 1027 centimeters across. It’s actually closer to 1029 centimeters across.