Our Universe

Given all that, how do we know the universe as a whole is flat? One way is to look at the distribution of galaxies in the universe.  One principle of cosmology is that on large enough scales the universe is homogeneous and isotropic. In other words, galaxies are distributed fairly evenly, and the universe looks pretty much the same in all directions.  That isn’t true on the scale of galaxy clusters, but on a cosmic scale across billions of light years it seems to be true.  If the universe wasn’t flat, then the light from distant galaxies would be distorted similar to gravitational lensing, but on a cosmic scale. We don’t see such an effect, so it seems the universe is flat. A more precise test of cosmic flatness uses the cosmic microwave background (CMB). From fluctuations in the CMB we can determine both the Hubble constant of the universe and the mass density. From these we can calculate the flatness of the universe. What we find is that to the limits of observation, the universe appears to be flat.

Although we know with high certainty that the universe is flat (or very, very close to flat), we still aren’t sure why it’s flat. It is sometimes referred to as the flatness problem, and it remains an unsolved mystery in cosmology.

The   Hologram   Cosmos

There has been a flurry of news articles about a new experiment that could prove we live in a two-dimensional hologram. Needless to say, we do not live in a 2-D hologram, and even if successful this new experiment would prove nothing of the sort. Unfortunately the “universe is a hologram” headlines always make great link-bait, and it doesn’t help that the press release for this experiment uses a similar link-bait headline. That said, the experiment is is very real, and if it succeeds it could revolutionize our understanding of the cosmos, so it is worth talking about.

The experiment in question is known as the Holometer, being run at Fermilab, and its goal is to detect very small fluctuations in space itself.  In physics there are two main theoretical frameworks to describe the universe: general relativity and quantum mechanics. Both of these theories have been verified numerous times by experiment, and both are extraordinarily accurate descriptions of their respective regimes. The problem is that each of them paint the universe in very different ways.

In general relativity, objects are solid and continuous. Space and time can be warped by the presence of mass, and can in turn cause masses to deviate from their normal, linear paths. General relativity is a classical theory, using many of the same assumptions about physical objects that Newton did in the 1600s. Quantum mechanics, on the other hand, proposes that objects are not solid. Instead they possess a duality of particle-like and wave-like characteristics. Quantum objects are typically described within a space and time that is fixed and unaffected by things like mass.

For large objects like apples and planets, you don’t typically need to worry about their quantum nature, so the assumptions of general relativity are perfectly fine. For small objects like atoms, you don’t typically need to worry about gravity, so the assumptions of quantum mechanics are fine. The problem comes with things that are both massive and small, such as black holes and the earliest moments of the big bang. In those cases we aren’t sure where the assumptions break down, and trying to figure out the physics gets problematic at best.

It’s generally thought that at some point the quantum nature of space and time can’t be ignored. This presumes that general relativity must give way to a quantum description of space and time. Two main approaches to quantum gravity are string theory (which generalizes particle physics to include gravity) and loop quantum gravity (which strives to quantize general relativity directly). One idea that seems central to both of these approaches is known as the holographic principle, from which all the “universe is a hologram” statements arise.

The holographic principle states that the information contained within a region of space can be determined by the information at the surface that contains it. For example, suppose there is a road 10 miles long, and its is “contained” by a start line and a finish line. Suppose the speed limit on this road is 60 mph, and I want to determine if a car has been speeding. One way I could do this is to watch a car the whole length of the road, measuring its speed the whole time. But another way is to simply measure when a car crosses the start line and finish line. At a speed of 60 mph, a car travels a mile a minute, so if the time between start and finish is less than 10 minutes, I know the car was speeding. The holographic principle applies this idea to quantum gravity. Just as its much easier to measure the start and finish times than constantly measure the speed of the car, it is much easier to do physics on the surface “hologram” than it is to do physics in the whole volume.

If the holographic principle is correct, then (so the Holometer team argues) there should be quantum fluctuations within space itself due to its dual nature. This would produce a background of “holographic noise” that could in principle be detected. The Holometer team hope to detect this quantum noise over the next few years.

It should be noted that this experiment is somewhat controversial. Theoretical calculations don’t clearly support the existence of holographic noise, and observations of gamma ray bursts seem to disprove its existence at a level detectable by the Holometer experiment. This is really cutting edge science, so it’s difficult to predict what the outcome will be.

What we do know for sure is that if the project is successful there will be lots of headlines declaring that the universe is a hologram. They will be wrong. It would just be the first direct detection of the quantum nature of gravity, which we’ve long suspected but haven’t been able to prove.