Categories: Cosmology

The Universe is the Same, Everywhere We Look. Even More than Cosmologists Predicted

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.

The edge of the observable universe mapped onto a celestial sphere. Credit: Planck Collaboration

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.

Large surveys of the cosmos show it is homogeneous. Credit: 2dF Galaxy Redshift Survey

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.

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.

Part of the KiDS map, showing variations in cosmic density. Credit: B.Giblin, K.Kuijken and the KiDS team

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).

Brian Koberlein

Brian Koberlein is an astrophysicist and science writer with the National Radio Astronomy Observatory. He writes about astronomy and astrophysics on his blog. You can follow him on YouTube, and on Twitter @BrianKoberlein.

Recent Posts

New Study Suggests Black Holes Get their “Hair” from their Mothers

Despite decades of study, black holes are still one of the most puzzling objects in…

12 minutes ago

Gaze at New Pictures of the Sun from Solar Orbiter

74 million kilometres is a huge distance from which to observe something. But 74 million…

27 minutes ago

Are Fast Radio Bursts Caused by Interstellar Objects Crashing Into Neutron Stars?

Astronomers have only been aware of fast radio bursts for about two decades. These are…

5 hours ago

Here’s How to Weigh Gigantic Filaments of Dark Matter

How do you weigh one of the largest objects in the entire universe? Very carefully,…

7 hours ago

How Could Astronauts Call for Help from the Moon?

Exploring the Moon poses significant risks, with its extreme environment and hazardous terrain presenting numerous…

20 hours ago

There Was a 15 Minute Warning Before Tonga Volcano Exploded

Volcanoes are not restricted to the land, there are many undersea versions. One such undersea…

20 hours ago