# The universe may not be endless, new research finds

When this mysterious number changes from 5.7 to 6, our guessaty about the shape of the universe changes… Physicists used to have many imaginations about the shape of the universe. Now, a new study, analyzing the Planck satellite, suggests that the universe may be a closed three-dimensional sphere.

A map of the shape of space (Photo: quantamagazine)

This overturns the prevailing view in recent years that the universe is an endlessly flat three-dimensional space. And this new conclusion involves a key number that determines the shape of the universe: the critical density of the universe.

3 Possibilities of Space

What is the shape of the universe? Does the universe have boundaries? For the contours of the universe, man has many illusions. But after Einstein applied the principles of cosmology to cosmology, the shape of the universe was severely limited. The cosmological principle requires that every point in space be equal, so space cannot have a very strange topology – to cite a counterexample, the universe cannot grow into a bicycle inner tube, because the point on the outer side of the inner tube is obviously not the same as the inner point position.

So, given the limitations of the cosmological principles, there are only three possibilities left for the shape of a connected universe: if space has a positive curvature, then it is a closed three-dimensional sphere; Then it is equally marginal, is a three-dimensional saddle face.

So, in this sense, the shape of space is determined by curvature. According to Einstein’s general theory of relativity, the curvature of space depends on the distribution of the field of matter in the universe. So, as long as the distribution of the field of matter in the universe is measured, the shape of space in the universe can be inferred.

Now, planck satellites provide a new basis for solving this problem. In a new paper published in Nature Astronomy, Eleonora Valentino of the University of Manchester and Alessandro Melchiorio of the University of Sapienza, Italy And Joseph Silk of the University of Oxford calculated a positive curvature of space based on the gravitational lens amplitude of the satellite’s cosmic microwave background radiation power spectrum. In other words, space is a closed three-dimensional sphere.

Critical density is key

So how do physicists judge the shape of space in the universe?

For space, there is a dividing line that determines its shape, which is the critical density of the universe.

In theory, we can define a critical density. This critical density is related to the Hubble constant, and its physical meaning is the average density of all matter and energy in the universe, including dark matter and dark energy. At this average density, space is flat.

Based on the Hubble constant, this critical density is equal to about 5.7 proton mass per cubic meter.

By contrast, we know that in the solar system, the density of dark matter alone has a mass of 30,000 protons per cubic meter. Therefore, the density of the solar system is much higher than the critical density of the universe. But because the distribution of matter elsewhere in the universe, such as interstellar space, is thin, the average critical density is certainly not as high as in the solar system. It is important to note that although the density of matter in the solar system is high, it is certainly not flat space in a local sense, but on a cosmological scale, we do not have to consider the size of the solar system, in cosmology, the Milky Way is a matter of mass. The discussion of whether space is flat is considered on a cosmological scale.

Thus, once the measured total density of the universe is equal to the critical density, the universe is flat three-dimensional Euclid space;

In these three cases, only the enclosed three-dimensional sphere indicates that space is limited, and in the other two cases the entire space is infinite.

Planck satellite gives new conclusions

One of the most important experimental methods is the observation of cosmic microwave background radiation. The study of cosmic microwave background radiation has won the Nobel Prize in Physics three times in 1978, 2006 and 2019. But this field is still in its infancy, and for humans, the study of the cosmic microwave background is an important means of understanding the universe.

A recent paper by Valentino et al. analyzed data from the Planck satellite that observes microwave background radiation in the universe. Planck’s telescope measures the average density of the universe by measuring the “gravitational lensing” of cosmic microwave background radiation over the past 13.8 billion years. Specifically, they are studying these cosmic microwave background radiant photons. The more material these photons encounter as they fly toward sending to Earth, the less clearly they will be in their direction that reflect their starting point in the early universe. In satellite observations, such phenomena have a vague effect. According to their analysis of the data, the average density of the universe may be 5% higher than previously estimated critical density. In other words, there is an average of six protons per cubic meter in the universe, rather than 5.7.

Cosmic microwave background radiation

According to the study, space may be a closed three-dimensional sphere. Prior to that, based on observations of cosmic microwave background radiation by the WMAP satellite and the first data from the Planck satellite in 2013, physicists established a cosmological standard model called LAMDA-CDM. In that model, the universe is flat. We can therefore say that the latest research presents a subversive conclusion.

The so-called three-dimensional sphere can also be associated with the mathematically famous Pongalai guess, which assumes that there is only one single-connected borderless three-dimensional geometry, the only possibility being a three-dimensional sphere. So, if Eleonola Valentino’s paper is correct, then we can link the Pongale guess to the shape of space, which has a theoretical aesthetic.

Of course, we must distinguish between the observable universe and the universe as a whole. The observable universe is just one space region of the entire universe, and we currently define earth as an ideal sphere with a radius of about 46 billion light-years.

In their latest paper, the researchers discuss the shape of the entire universe, arguing that there is a 99 percent chance that the universe is more than 99 percent likely to be a closed three-dimensional sphere.

Despite the latest results from the study, the debate over the shape of space in space has yet to settle. One important reason is that the calculation of the critical density of the universe depends on the measurement of the Hubble constant. But for now, the Hubble constant is uncertain: although planck gives a Hubble constant, it is clearly at odds with other methods of measuring the Hubble constant. Since hubble’s constant is unpredictable, the average density of the universe is not accurate, so this threshold is blurred. Therefore, it is too early to say that space must be closed.

The Simmons Observatory, currently under construction in Chile, and the Ali Cosmic Microwave Background Radiation Polarization Detector in Tibet, China, will provide more accurate measurements. Perhaps in the next five years, we will be able to re-understand this space from the observations of these devices.