# Q & A: Could Dark Energy Really Be Gravity?

Q:
Could is not be that dark energy is really just gravity? Could the universe be creating matter as it expands? Perhaps in a process similar to how black holes radiate? I would expect that if the big bang were an explosion you would see more matter toward the center and less matter away from the center. But we observe uniform matter as we look out into the Cosmos. If the universe is creating matter as it exands the extra gravity created at the periphery could cause matter inside the universe to accelerate in expansion and would also explain uniformity of matter.
- Matt Collins (age 42)
St. Petersburg, FL, USA
A:

Hi Matt,

That's a great question! Much of what you describe is not unlike the famous (but now discredited) . But before we get into that, let's cover the basics:

We know the Universe is expanding (Edwin Hubble discovered this in 1929 after measuring redshifts of distant galaxies), and we also know that its rate of expansion is actually accelerating (a 1998 discovery that recently earned astrophysicists Saul Perlmutter, Brian P. Schmidt, and Adam G. Riess a Nobel Prize!). This acceleration is very much contrary to what we would expect because our Universe contains lots of matter and every bit of that matter attracts every other bit gravitationally. This attractive force should slow expansion. Clearly, either something within our existing model is reversing this effect and acting as "antigravity" (Dark Energy is the name we've given the energy density that would produce this effect) or that model needs to be modified to account for this anomalous observation. We're fairly certain it's the former.

It sounds like you are somewhat familiar with the Cosmological Principle. This is the idea that, on large enough scales, the Universe is both homogenous (of roughly the same density from one region to the next) and isotropic (having basically the same properties in all directions). One consequence of this principle is that the Universe cannot have special points like a center or edges of any sort, so there is no "periphery" at which matter could be created as you described. And, even if there were, it would just compound the attractive effect I mentioned earlier.

The Steady State Theory employs a more extreme version of the Cosmological Principle (deemed the "Perfect Cosmological Principle") which asserts that the Universe is homogenous in space and in time. If this were true, the expansion of our Universe would necessitate the continuous creation of matter to keep its mass density constant. The creator of the SST, Fred Hoyle, proposed that this matter was replenished by something he called the "C-field" which might also account for the expansion of the Universe.

There was a time when scientists were legitimately divided between the Steady State Theory and Big Bang Theory, but we have since seen direct evidence that the Universe changes in time. Observations of distant quasars and radio galaxies indicate that they only existed long, long ago, and the – relic radiation from the Big Bang – fills the observable Universe and offers us a glimpse into the distant past. The Universe is emphatically not homogeneous in time.

As for matter being created in much the same way as black hole ("Hawking") radiation, remember that Hawking's theory relies upon the presence of a horizon of sorts (the "event horizon" for black holes), and that the existence of such a special point would violate the Cosmological Principle.

In case there was any doubt, it seems worth concluding with the fact that the Cosmological Principle is not just some abstract theory. Observations show that the Universe is statistically homogenous on scales larger than 250 million light years, and the cosmic microwave background is remarkably isotropic. If you're interested, you can read more about it .