The idea of a mirror universe is a common metaphor in science fiction. A world similar to ours where we might find our evil doppelganger or a copy of us who actually asked about our high school crush. But the concept of a mirror universe has often been studied in theoretical cosmology, and as a new study shows, it may help us solve problems with the cosmological constant.
The Hubble constant, or the Hubble parameter, is a measure of the expansion rate of our universe. This expansion was first demonstrated by Edwin Hubble, using data from Henrietta Leavitt, Vesto Slipher, and others. Over the next several decades, measurements of this expansion stabilized at a rate of about 70 km/s/Mpc. Give or take a little. Astronomers have discovered that as our measurements become accurate, the different methods will settle on a common value, but this has not happened. In fact, the measurements in the past several years have become so accurate that they differ completely. This is sometimes known as The problem of global tension.
At this point, the observed values of the Hubble constant cluster are divided into two groups. Measurements of fluctuations in the cosmic microwave background indicate a lower value, around 67 km/s/Mpc, while observations of objects such as distant supernovae give a higher value around 73 km/s/Mpc. Obviously, something adds nothing, and theoretical physicists are trying to figure out why. This is where the mirror universe might come in.
Wild ideas tend to fall and fall out of popularity in theoretical physics. The idea of a mirror universe is no exception. It was little studied in the 1990s as a way to deal with the matter and antimatter symmetry problem. We can create matter particles in the lab, but when we do that, we are also creating antimatter particles. They always come in pairs. So, when particles formed in the early universe, where did their antimatter siblings go? One idea was that the universe itself formed as a pair. The matter of our universe and the universe of antimatter are similar. The problem has been resolved. The idea hasn’t been accepted for various reasons, but this new study looks at how to solve the Hubble problem.
The team discovered a consistency in what is known as . Non-uniform parameters. The most famous of these is the fine-structure constant which has a value of about 1/137. Basically, you can combine the measured parameters in a way that cancels out all the units, giving you the same number no matter what units you’re using, which is great if you’re a theorist. The team found that when cosmological models are adjusted to match the observed expansion rates, many non-uniform parameters remain the same, suggesting an underlying cosmological symmetry. If you impose this symmetry on a larger scale, you can measure the rate of free fall of gravity and the rate of photon and electron scattering so that the different methods of Hubble measurement agree better. And if this constancy is real, then it implies the existence of a mirror universe. One that would affect our universe through faint gravity.
It should be noted that this study is mostly proof of concept. It shows how this cosmological constant might solve the Hubble constant problem, but it doesn’t go so far as to prove that it is a solution. A more detailed form will be needed for this. But it’s an interesting idea. And it’s good to know that if your evil doppelganger exists, it can only affect your life with an attractiveness…
Reference: Cyr-Racine, Francis-Yan, Fei Ge, and Lloyd Knox. “Cosmic observatories symmetry, a dark sector of the mirror world, and the Hubble constant. ” Physical Review Letters 128.20 (2022): 201301.