The mystery of the “cold spot” remains – Void can only partially explain the abnormally cold spot in the cosmic background –

Mysterious anomaly: Astronomers have examined a possible explanation for the enigmatic cold spot in the cosmic microwave background – but even they can’t solve the mystery completely. Accordingly, a gigantic void in the foreground causes at least part of the detected anomaly. Because the gravitation is lower there due to the lack of mass, the light of the background radiation is stretched less. But that alone is not enough.

All of space is permeated by cosmic background radiation—ubiquitous microwave radiation that dates back just 380,000 years after the Big Bang. The temperature fluctuations of this radiation reflect subtle differences in the distribution of energy and mass in the early Universe. In addition, they allow conclusions to be drawn about the proportions of dark matter and dark energy as well as the expansion rate of the universe.

The section shows the cold spot in the Planck map of the background radiation, the rest shows the distribution of dark matter in this area. © Gergö Kránicz and András Kovács

Cold spot is a puzzle

But there is one spot in the microwave background that doesn’t fit the cosmological picture: the cold spot. This zone, which extends over ten degrees, is on average around 70 millikelvin colder than the rest of the background. It therefore appears in current maps of background radiation as a distinct blue zone in the southern hemisphere of the sky. This cosmic anomaly is significantly larger and more pronounced than the standard cosmological model suggests.

But what is the cause? So far there is no clear answer. Some hypotheses such as measurement errors and interference effects from Milky Way structures have already been largely refuted. Others, including still unrecognized processes in the early cosmos or exotic phenomena such as the signature of parallel universes, remain open.

Primordial photons and the Sachs-Wolfe effect

“But there is also an active debate that possible physical processes in the cosmic foreground can produce such a spot on the microwave background,” explain András Kovács of the Dark Energy Survey collaboration and his colleagues. One of these effects is the so-called Integrated Sachs-Wolfe Effect (ISW). This occurs when photons fly through a zone of lower or higher matter density on their way from distant space to us.

The reason: according to Einstein’s general theory of relativity, matter also influences the behavior of light through its gravitational pull. When photons fly through an area of ​​increased gravity, their waves are stretched and red-shifted. On huge scales, however, the expansion of space also comes into play: it ensures that galaxies and other matter are pulled apart and the gravitational sink flattens out as a result.

If a photon now crosses such a large zone that the flight takes millions of years, an imbalance occurs: it is deeper when it enters the gravitational sink than when it exits. “As a result, the light particles experience a net loss of energy on their journey, which is known as the integrated Sachs-Wolfe effect,” explains co-author Juan Garcia-Bellido from the University of Madrid.

A void as author?

For the cold spot, this means that if the photons of the cosmic background radiation fly through an area with particularly rarefied matter on their way to us, this can contribute to their cooling and create an anomaly. This is one of the reasons why astronomers suspected as early as 2015 that an empty zone in front of the cold spot, the Eridanus Supervoid, could have caused it.

The problem, however: “Subsequent studies showed that the Eridanus Supervoid can only explain around 20 to 30 percent of the observed cooling – at least if you start from the cosmological Standard Model,” says the research team. However, the exact extent, shape and density of this gigantic void in the cosmos has so far remained a matter of debate.

Blanco Telescope
The Dark Energy Camera is located in the Blanco Telescope in the Chilean Andes.© Reidar Hahn/ Fermilab

Kovács and his colleagues therefore remeasured the Eridanus Supervoid and its possible ISW effect. To do this, they evaluated data from the Dark Energy Camera, an optical instrument that specializes in measuring cosmic structures and masses. From this data, they created a map of the mass distribution in the area of ​​the cold spot and the empty zone in front of it.

Largest anomaly in the known cosmos

The result: “The Eridanus Supervoid is one of the largest known thinned zones in the entire observable universe,” the astronomers report. Throughout the Dark Energy Survey’s mapping area, no void zone is as large or as pronounced as this one. The cigar-shaped supervoid spans 1.8 billion light-years and begins around two billion light-years from Earth. The matter density in it is around 30 percent lower than in the surrounding universe.

The decisive factor, however, is that the supervoid has a measurable effect on gravity and the ISW effect in this area of ​​the universe. According to the new data, the effect of the Eridanus Supervoid on background radiation is stronger than previously thought. At the same time, however, it deviates significantly from common models, as the astronomers report. In simulations based on the standard cosmological model, they came up with a deviation of around 30 percent.

Is the standard model wrong?

The astronomers see two possible explanations for this. If the current lambda CDM model of cosmology is correct, then the Eridanus Supervoid alone is not sufficient to explain the cold spot. Then the background radiation itself and thus a process in the early cosmos would at least have to contribute to this anomaly.

The second possibility is that the Standard Model is incomplete and supervoids cause a stronger ISW effect than it intended. Behind this could be a still unrecognized, stronger effect of dark energy and thus a faster cosmic expansion. “The problem, however, is that alternative models cannot explain this discrepancy either,” says Kovács. “If the effect is really real, then it could mean that we don’t yet understand something fundamental about dark energy.”

In other words, the cold spot and its possible causes keep raising new questions. “The effect of superstructures on the cosmic background remains an exciting and unsolved problem in cosmology,” the research team concludes. (Monthly Notices of the Astronomical Society, 2022; doi: 10.1093 / mnras / stab3309)

Quelle: Fermi National Accelerator Laboratory

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