No matter which direction you look in the Universe, the view is basically the same if you look far enough. Our local neighborhood is populated with bright nebulae, star clusters, and dark clouds of gas and dust. There are more stars toward the center of the Milky Way than there are in other directions. But across millions, and billions, of light-years, galaxies cluster evenly in all directions, and everything starts to look the same. In astronomy, we say the Universe is homogeneous and isotropic. Put another way, the Universe is smooth.
This doesn’t mean the Universe is perfectly smooth at large scales. Even at the most distant edge of the visible Universe, there are small fluctuations. Observation of the Cosmic Microwave Background (CMB) shows minor variations in temperature caused by areas of higher and lower density that existed in the early Universe. This is precisely what we would expect. In fact, the scale of CMB fluctuations allows us to measure dark matter and dark energy.
The distribution of galaxies also isn’t perfectly smooth. They are grouped into superclusters separated by voids of mostly empty space. The small density fluctuations seen in the CMB laid the seeds for galaxy clusters to form. According to the LCDM model of cosmology, early galaxies were drawn toward more dense regions. As the Universe expanded over billions of years, the current structure of superclusters and voids formed. Since the scale of CMB clusters gave rise to galaxy clusters, measurements of the CMB allow us to predict the size of superclusters. In other words, the level of smoothness in the early Universe makes a prediction about the smoothness of galaxy clusters we should see.
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But a new study of galaxies finds that our prediction doesn’t quite agree with what we observe. The Kilo-Degree Survey (or KiDS) has mapped more than 31 million galaxies within 10 billion light-years. The survey covers about half the age of the Universe and gives us the positions of these galaxies and their statistical “clumpiness.” Using the KiDS data, a team has found that galaxies are about 10% more homogeneous than predicted. The Universe is smoother than we thought, and it isn’t clear why.
While the result is clear, it isn’t particularly strong by rigorous scientific standards. There is a small chance that galaxies just happened to be more evenly distributed by random chance. But this result could also hint at some kind of new physics or flaw in our current cosmological model. There have been a few hints like this. Enough that astronomers are starting to look at alternatives.
But for now, the best option is to keep gathering data. The answer is out there, and with careful observations like these, we will eventually find it.
Reference: Heymans, Catherine, et al. “KiDS-1000 Cosmology: Multi-probe weak gravitational lensing and spectroscopic galaxy clustering constraints.” arXiv preprint arXiv:2007.15632 (2020).