Can the Euclid telescope solve the mystery of dark matter?

China Newsweek: Yang Zhijie

Published on July 2023, 7, the 10th issue of China Newsweek magazine

At 7:1 Beijing time on July 23, the Euclid Space Telescope (hereinafter referred to as the Euclid Telescope) designed by the European Space Agency (hereinafter referred to as ESA) was launched from Florida, USA, on a Falcon 12 rocket of SpaceX in the United States. Over the next 9 years, it will survey more than a third of the universe, do "CT scans" of more than 6 billion galaxies, and create a three-dimensional "map" of the universe.

According to the official website of ESA, based on the massive data observed by the Euclid telescope, it is possible to determine the expansion and structural evolution of the universe in the past 100 billion years as never before, and help uncover the two mysteries of cosmology: dark energy and dark matter.

Gong Yan, a researcher at the National Astronomical Observatory of the Chinese Academy of Sciences, has long studied dark matter and dark energy models, and is the executive deputy director of the Scientific Research Center of the National Astronomical Observatory of the China Sky Survey Space Telescope (CSST). He told China Newsweek that dark matter and dark energy are currently the most cutting-edge research fields in physics and astronomy, and they are also "two dark clouds" hanging over the heads of the two disciplines. Many countries are tackling problems in related fields, and if these two mysteries are solved, it will be a very major breakthrough for mankind.

Humans still know nothing about 95% of the universe

In 2009, ESA launched the Planck exploration satellite, which took three years to accurately map the microwave background radiation of the universe, which is the residual radiation from the Big Bang and is known as the oldest light in the universe. In 3, when the first panorama reflecting the early days of the universe was made public, scientists confirmed that it almost perfectly validated the Standard Model of the universe. That is, in the universe, all visible ordinary matter, including stars, planets and everything in our lives, accounts for only 2013% of the total energy of the universe, dark matter accounts for 5%, and dark energy accounts for 27%.

But the question that needs to be answered is what exactly is dark matter and dark energy. Over the past nearly century, many researchers have measured that stars are rotating faster around the center of galaxies than predicted, speculating that the presence of invisible matter increases the gravitational pull that spins stars, which scientists call dark matter. Dark energy, on the other hand, refers to the mysterious matter that drives the accelerating expansion of the universe. Dark matter is different from ordinary matter, which cannot be seen, touched, or heard. Over the past few decades, astronomers and theoretical physicists have tried their best to directly detect the existence of dark matter and dark energy, let alone know anything about their properties.

In 2012, the Euclid program was selected by ESA to be implemented, and more than 13,2000 scientists from 10 European countries, the United States, Japan and other countries participated in the program, spending about 1 billion euros to develop the Euclid probe. Now, the probe, named after the father of geometry, carries the 2.150-meter Euclidean telescope, as well as a visible-wavelength camera and a near-infrared camera, and will travel for a month in space, eventually hovering at L2, Lagrange point, <>.<> million kilometers from Earth. The Euclid telescope will share the same position as the James Webb telescope, gazing into the depths of the universe together.

The launch of the Euclid telescope went through several waves. In 2022, the construction of the Euclid telescope was completed, and it was originally planned to carry Russian rockets and launch from South America, but due to the impact of the Russian-Ukrainian war, the cooperation between the space departments of Europe and Russia was terminated. At the end of 2022, ESA found an alternative, partnering with the American company SpaceX to transfer the launch site to the United States. According to the goals announced on the official website of ESA, the Euclid telescope will be used to study the existence of dark energy, or to find out what is the reason for the obvious rapid expansion of the universe; If dark energy exists, what are its characteristics, and based on the observed large-scale structure of the universe, the state of the universe after the Big Bang is studied.

Gong Yan told China Newsweek that the Euclid telescope does not directly detect dark matter, strictly speaking, the detection of dark matter needs to detect dark matter particles. There are several theoretical hypotheses for dark matter, one of which holds that dark matter is a heavy mass particle. In the past few decades, scientists have detected dark matter in three main ways: using large colliders to test whether dark matter particles can be knocked out; Set up a laboratory more than ten kilometers underground to ensure that other particles cannot penetrate and look for traces of dark matter interacting with other substances; Common particles produced after the "annihilation" or decay of dark matter on the ground or in space. But so far, people have exhausted all the above methods, and no trace of dark matter particles has been detected.

Gong Yan explained that the working principle of the Euclid telescope is to assume the existence of dark matter, through the detection of a large number of celestial objects in the universe, the obtained data will be compared and verified with the relevant models to determine or exclude the corresponding dark matter model. This is actually a theoretical "elimination". "If people detect the properties of dark matter accurately enough, no other theory can explain this data, and finally it can only be explained by dark matter." Gong Yan said.

According to the official website of ESA, the Euclid telescope achieves scientific goals in two main ways. The telescope will measure 15.<> billion background galaxies, creating a three-dimensional view of the distribution of dark matter in the universe, which cosmologists will use to infer how galaxy structures formed in the history of the universe and how fast they have grown. This is related to the nature and quantity of dark matter and dark energy. At the same time, the Euclid telescope will also use baryon acoustic oscillations to measure the expansion rate of the universe and its changes. Baryon acoustic oscillations can be simply understood as sound fluctuations in the early universe, which left traces in the cosmic microwave background radiation, which helps to understand the evolution of the universe and the formation of its structure. The two methods of measurement are almost identical objects, and the results can be cross-checked to reduce errors.

As a fourth-generation survey telescope, Euclid and Hubble have a clear difference. "Hubble is a precision telescope that can clearly photograph objects in a small area of the sky, and can only detect a small area in space, which is equivalent to observing through a 'pinhole'." Gong Yan said that to study dark matter and dark energy, telescopes need to see the wider universe. The Euclid telescope "removes the pinhole" and scans the entire universe in a larger field of view.

In addition, the Euclid telescope can also "see farther." The light emitted by celestial bodies is stretched as the universe expands, and a "redshift" occurs. The greater the redshift, the farther away the celestial body is from humans. Euclid also carried a visible light camera that measured the shape of galaxies and a near-infrared spectrometer that measured the brightness and distance of galaxies, which could observe light 30 billion years after the formation of the universe. "It detects the breadth and depth of the universe that no other telescope can match at present, and the amount of observation data is unprecedented in the same type of telescope." Gong Yan told China Newsweek.

Another important scientific goal of the Euclid telescope is to measure the mass of neutrinos through the analysis of the structure of the cosmic matter distribution. At present, scientists believe that neutrinos are composed of three different types of neutrinos: electron neutrinos, muon neutrinos, and tao neutrinos, and existing ground experiments cannot confirm the mass order of the three. In the Standard Model of particle physics, neutrinos theoretically have zero mass, but ground-based experiments and observations show that neutrinos have mass. "Accurate measurement of the sum of the masses of the three neutrinos plays an important role in understanding the mass sequencing of neutrinos and the mechanism by which they acquire them." Gong Yan analyzed that this will greatly promote the study of neutrinos and even basic physics.

The observation target cannot be completed alone

On Earth, light travels in a straight line in a vacuum, but in the wider universe, this is not the case. According to Einstein's general theory of relativity, light emitted by a distant galaxy passes through massive objects such as galaxies or clusters of galaxies, and is affected by gravity and will bend slightly. Similar to being placed under a magnifying glass, the imaging morphology and brightness are changed, which is called the gravitational lensing effect. Scientists believe that the distortion of the background light source is not random, it is related to the gravitational field of dark matter, from which the density distribution of dark matter can be detected.

Gong Yan explained that ideally, gravitational lensing is strongest when light sources such as luminous galaxies, massive matter passing by light, and the observer's three-point line. But in the universe, under normal circumstances, only a weak lensing effect occurs in the light source, making observation more difficult, which is called weak gravitational lensing. After officially opening the detection, the Euclid telescope will map the distribution of dark matter through the weak gravitational lensing effect. "Although the shape and brightness of individual galaxies change very little, this is a statistical concept, and if you measure the shape and brightness of hundreds of millions or billions of galaxies, the statistical effect will come out." Gong Yan said.

Therefore, the Euclid telescope has to observe a sufficiently wide universe, Gong Yan said, "In terms of design, the Euclid telescope uses only a wide band for visible light observation, which helps to measure the shape of galaxies more accurately." "But large survey telescopes generally use photometric redshift technology, which uses metering data from multiple bands to estimate redshift. Euclid uses only one visible wide band and a few near-infrared bands, which reduces the accuracy of metering redshift, "redshift measurement is inaccurate, and the measurement of weak gravitational lensing effects cannot be very accurate." Gong Yan said.

This is one of the biggest challenges facing the future detection of the Euclid telescope. Gong Yan introduced that if you want to successfully complete the scientific goal, the Euclid telescope needs the cooperation of other telescopes. The ESA website also mentions that the telescope needs additional data from ground-based telescopes to improve redshift measurement techniques and modeling the point spread function of individual galaxies. However, Gong Yan mentioned that how different telescopes can coordinate observation and data processing will be a very big challenge for the measurement of the Euclid telescope. For example, the challenges to be faced include the vulnerability of ground-based telescope observations to atmospheric disturbances, the different results of each observation, and the different instrument parameters.

In recent years, Europe, China, the United States and other countries and regions are committed to the research and detection of dark matter and dark energy. NASA is developing the Nancy Grace Roman Space Telescope, which is scheduled to launch in 2027. China's two-meter space survey space telescope (CSST) is scheduled to launch in 2024. These telescopes have similar research directions to Euclid, used to carry out wide-area sky survey observations, and carry out cutting-edge research in the fields of cosmic structure formation and evolution, dark matter and dark energy.

In 2013, the China Sky Survey Space Telescope Program was approved, and Zhan Hu, a researcher at the National Astronomical Observatory of the Chinese Academy of Sciences, participated in the development of CSST, telling the media that CSST was scheduled to be delivered before the end of 2023 and planned to be launched in 2024. According to reports, the CSST spacecraft is the size of a bus and stands 3 stories tall. CSST's main mission is the CSST, which will cover 40% of the entire sky and accumulate high-quality data on nearly 20 billion galaxies.

Among the many scientific goals of NASA's Nancy Grace Roman Space Telescope, it also includes studying cosmic scales and dark energy, and it will also search for cosmic supernova events. But it is different from Chinese and European astronomical telescopes. According to ESA, the mission of the Roman and Euclid telescopes is "complementary", Euclid's measurement field is wider, while Roman only surveys a smaller area, but will detect with higher resolution and greater wavelength coverage, and the overlapping results of the two can be used to check each other's systematic errors.

Can such survey telescopes finally lift the veil on dark matter and dark energy? Isabel Hooke, a professor of astrophysics at Lancaster University in the United Kingdom and a scientist at Euclid, mentioned in an interview that Euclid will send back a photo of its "first impression" of the universe in a few months, but scientists will have to wait years before they can get a huge amount of data and announce some new results.

China Newsweek, Issue 2023, 25

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