© Forbes
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We all know that the broadest theory currently explaining the origin of the universe is the Big Bang. Based on the Big Bang theory, it's easy to assume the shape of the universe based on the inertia of the mind, and conclude that the universe is a sphere—the universe doesn't seem to have any reason to exhibit some asymmetry.
This is the "logical comfort zone" that every thinker who is searching for objective truth tries to avoid. Even if the universe is really a sphere, we need to prove this conclusion with enough evidence, not take it for granted. For now, we are still far from that goal.
When we say that the shape of an object is expressed by default in a three-dimensional spatial model. But many scientists tell us that the universe is far more than 3 dimensions, so how do we understand the shape in a space higher than three dimensions? What the relationship between the number of shapes and dimensions should be. If possible, please advise.
As shown on the left side of the image above, if it is in a flat universe, a straight line will extend to infinity; while the closed universe on the right side of the picture is like a sphere, with a surface of surfaces. In a closed universe, a straight line will eventually return to its starting point. © Lucy Reading-Ikkanda
Recently, the journal Nature Astronomy published a controversial paper. The paper questions the "universe is flat" predicted by standard cosmological theory, arguing that the universe should be curved and closed like a sphere. The authors re-analyzed the main cosmological dataset and concluded that this data underpins the conclusiveness of closed cosmology by 99 percent — even though there is still other evidence that the universe is flat.
（www.nature.com/articles/s41550-019-0906-9）
Alessandro Melchiorri of the University of Rome I points out that the data mentioned here is an observation of ancient light by the Planck Space Telescope, known as the Cosmic Microwave Background (CMB). He believes the data "clearly points to a closed model of the universe." He co-authored the paper with Eleonora di Valentino at the University of Manchester and Joseph Silk of the University of Oxford. They argue that the cosmic microwave background data suggest that the universe is closed; this inconsistency with other data that underpins the cosmos flatness theory is a "cosmic crisis" that requires "rethinking."
Cosmic microwave background image of the Planck satellite. © ESA /Planck Collaboration
But the team behind planck's telescope came to a different conclusion in a 2018 analysis. Antony Lewis, a cosmologist at the University of Sussex, says that some specific features in the cosmic microwave background data would be used as an argument for a closed cosmology by di Valentino et al., which is a simple data accident — and that's the simplest explanation. Lewis and other experts note that they have taken a closer look at the issue and the relevant doubts in the data.
（arxiv.org/abs/1807.06209）
Graeme Addison, a cosmologist at Johns Hopkins University who was not involved in Planck's analysis and the paper, said: "There is no doubt that these phenomena exist objectively to some extent, it is just that people disagree on how to interpret them." ”
Is the universe flat? If two beams of light were emitted side by side in space, would they remain parallel forever, or would they eventually cross in a closed universe and return to where they began? The answer to the question depends largely on the density of the universe. If the density of all matter and energy in the universe (including dark matter and dark energy) adds up just enough to balance outwardly expanding energy with inward gravity, the universe would flatten in all directions.
The shape of the universe depends largely on the density of the universe. © www.learner.org
The theory of cosmic inflation is the dominant theory of the birth of the universe, which holds that the universe is flat. Since the beginning of the 21st century, various observations have shown that our universe is almost flat, and its density must be within this critical density range. This critical density range can be calculated as about 5.7 hydrogen atoms per cubic meter of space, most of which are invisible.
Planck measures the density of the universe by measuring the deflection, or "gravitationally lensed," that has occurred as the cosmic microwave background light passes through the universe over the past 13.8 billion years. The more material these photons in the cosmic microwave background encounter on their way to Earth, the more deflections occur, and their orientation does not clearly reflect their starting point in the early universe.
This phenomenon manifests itself in the data, which is a blur effect that eliminates some of the peaks and troughs in the light space pattern. In the new analysis, a large number of deflections in the cosmic microwave background suggest that the density of the universe may be about 5% higher than the critical density, with an average density of about 6 hydrogen atoms per cubic meter instead of 5.7 hydrogen atoms. Higher densities would give gravity the upper hand, making the universe a closed whole.
Planck's team noticed that years ago the deflection effect of light was greater than previously expected. In the final analysis of the full data set released last year, anomalies were the most pronounced. If the universe were flat, cosmologists would expect measurements of cosmic curvature to fall within a "standard deviation" of 0, given the random statistical fluctuations in the data.
But both Planck's team and the paper's authors found that the data in the cosmic microwave background deviated by a full 3.4 standard deviations. If the universe is really flat, then it's a big "fortuitous event," roughly equivalent to 11 consecutive heads-ups when tossing a coin. The probability of this happening is less than 1%. Planck's team attributed the measurement to chance, or rather to an unexplained effect that, like the effect of extra matter, obscures the cosmic microwave background light.
It's also possible that the universe is really closed. Di Valentino et al. point out that closed models of the universe could explain other anomalous findings in the cosmic microwave background. For example, researchers measure the color change in the cosmic microwave background light from different regions of the sky to infer the values of key components of the universe, such as dark matter and dark energy. Curiously, when the size of the area they used for comparison changed, so did the answers they got. Di Valentino et al. point out that if these values were recalculated with a closed model of the universe, they would not be any different.
Will Kinney, a cosmologist at the University at Buffalo in New York, thinks this benefit of a closed model of the universe is "very interesting." But he also pointed out that the difference in the size of the variable seen in the cosmic microwave background light is likely to be just a statistical fluctuation, or an unrecognizable error that affects the deflection measurement.
A pie chart showing the energy components of different matter in the universe, about 96% of the energy comes from strange dark matter and dark energy. © iVoox
In the standard theory of cosmology, there are six key properties used to describe the universe, the so-called "ΛCDM" model of the universe (named after dark energy — denoted by the Greek letter Λ — and cold dark matter). The ΛCDM model of the universe can accurately describe almost all the characteristics of the universe with only 6 parameters. According to the ΛCDM model of the universe, the universe does not have any curvature, the universe is flat.
"The point is not that the universe is closed. The focus is on inconsistencies between data. ”
—Alessandro Melchiori, University of Rome I
The paper by Di Valentino et al. rightly points out that we may need to add a seventh parameter to the ΛCDM model to describe the curvature of the universe. This parameter improves the fit of the data in light deflection measurements.
But other cosmologists argue that before taking this anomaly too seriously and adding a seventh parameter to the ΛCDM model, we must take into account all other cases in which the ΛCDM model is correct. Of course, we can also focus on this anomalous phenomenon, like 11 consecutive coin tosses with heads up, insisting that there is something wrong with the model. But the dataset of cosmic microwave backgrounds is so vast that it's like flipping a coin hundreds or even thousands of times. It's hard to imagine that in a situation like this we would randomly encounter 11 consecutive heads-ups. Physicists refer to this phenomenon as the "look elsewhere" effect.
The researchers also note that most other measurements do not require a 7th parameter. There is a second method that can collect the spatial curvature of the cosmic microwave background by measuring the correlation between four points of light from the sky; this "lensing reconstruction" measurement also shows that the universe is flat and does not require the addition of a seventh parameter.
ΛCDM model, accelerating the expansion of the universe. © The Great Courses Plus
Independent observations of cosmic signals (i.e., baryon acoustic oscillations) from baryon vibration spectroscope surveys also suggest that the universe is flat. In his 2018 analysis, Planck combined the deflection measurements with two other measurements to arrive at a mean of 0 for the spatial curvature and 1 standard deviation.
Di Valentino et al. argue that putting these three different datasets together obscures the fact that different datasets are actually inconsistent. Melchiori wrote in an email: "The point is not that the universe is closed. The focus is on inconsistencies between data. This suggests that our current model isn't exactly the same as the real universe, and we're missing something. In other words, the ΛCDM model is wrong or incomplete.
Other researchers were consulted during the writing process, who all agreed that there was enough evidence that the universe was flat. Addison said, "Considering other measurements, the statistical fluctuations are the most obvious explanation for the existence of this phenomenon in the Planck data." It could be caused by some lack of precision in Planck's analysis, or it could be entirely noise fluctuations or random accidents. But either way, there's no reason to take this model of the closed universe seriously. ”
But that's not to say that there aren't missing pieces in existing cosmological grand plans. The current rate of expansion of the universe predicted by the ΛCDM model seems to be wrong, which has also caused a lot of controversy, the so-called Hubble constant problem. However, the theory of cosmic closure does not really solve this problem. In fact, increasing the curvature of this parameter only makes the prediction of the inflation rate worse. Aside from rejecting Planck's anomalous deflection measurements, there is no reason to believe that the universe is closed.
Kinney said: "Time will tell, but I personally don't worry about it. "He's not worried about curvature in the cosmic microwave background data." It is no different from anomalies that have proved meaningless. ”
Text:Natalie Wolchover
Translation/Yang Rui
Proofreading/Rabbit's Lingbo microstep
Original/www.quantamagazine.org/what-shape-is-the-universe-closed-or-flat-20191104/
This article is based on a Creative Commons License (BY-NC) and is published by Rui Yang in Leviathan
The views of the author are only those of the author and do not necessarily represent leviathan's position
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