This Week’s Science News from SWTG

Geothermal Energy & Our Lopsided Universe

Our Universe Has Two Different Sides, Physicists Confirm

Image: Sanyal et al, arXiv:2411.15786

The universe is an odd place, and it just became even odder. Because a new analysis confirmed an anomaly that everyone ignored because they thought it would go away. Well, it didn’t. It seems that the universe looks noticeably different to one side of us than on the other. Let’s have a look.

This anomaly is called the Hemispherical Power Asymmetry. It’s been around since 2003, when WMAP data showed that the cosmic microwave background has more fluctuations on one side of the universe than on the other. If that makes no sense to you, it’s because it makes no sense.

This asymmetry quite simply shouldn’t be there. In fact, I don’t know any good explanation for it, which is probably why physicists have largely ignored it. There isn’t even a theory for it. It’s just weird.

The Cosmic Microwave Background, CMB for short, is radiation that is left over from the hot plasma in the early universe and that is now all around us. The radiation was once hot, but has since dropped to an average temperature of about just about 2.7 Kelvin.

However, the temperature isn’t exactly the same in all directions in the sky. This is because in this hot plasma in the early universe, some places were a tiny little bit hotter and others a tiny little but colder. So the cosmic microwave background has temperature fluctuations around the average.

Physicists have studied these temperature fluctuations intensely because they’re our best clue to what was going on after the Big Bang. Loosely speaking, they count how many patches of which size are hidden in this image. What you can see by eye is that there are a lot of patches that are kind of this size, which is about 1 degree in angular size. But really there are fluctuations of any size, it’s just that they’re difficult to see.

The Hemispherical Power Asymmetry is now the observation that one side of the universe seems to have more temperature fluctuations than on the other side.

This is not about the CMB temperature itself, it’s about the fluctuations. The CMB temperature itself is also somewhat larger in one direction than in the other, but this is because of our own motion through the universe. It’s called the CMB dipole, and is more or less well-understood.

But the hemispherical power asymmetry is something different altogether. It means, loosely speaking, that there is more texture or detail on one side of the universe than on the other, even if the average is the same.

However, 20 years ago the data wasn’t particularly good. The new paper now looked for the anomaly again in a much newer data set, from the Planck mission. For their analysis, they divided up the entire sky into increasingly smaller patches and then calculated the typical size of fluctuations in them. From this, they then extract the two directions that are most different and also calculate how different they are.