Surveys

Surveying objects in the sky is the core work of astronomers. We can survey the sky for the highest energy light, gamma rays and X-rays, through ultraviolet (UV), visible and infrared (IR) light, to the lowest energies of light, the microwaves and radio waves. Earth’s atmosphere absorbs some of these wavelengths and it also blurs the light that it does let through. For this reason, telescopes are on Earth, some are placed on a balloon or a plane to be within a thin atmosphere and some are in space so that there is no atmosphere to interfere with the measurements. There is even a gold-plated telescope on the moon, placed there during the lunar landing in 1972, that took photographs of UV light and was used to show that interstellar gas (the gas between stars) is made up of molecules of hydrogen. In 2013, China sent another UV telescope to the moon (Chang’e-3) which beams back data on galaxies and hot bright objects.

Satellite telescopes are the most expensive telescopes but for some wavelengths they are the only option. Balloon-based telescopes are used for surveys that must be above the atmosphere and are cheaper than satellites. Ground- based telescopes are the most popular and there are many of them all over the World, often in areas where the air is dry (deserts) or where the air is thin (mountains), or both, which helps to reduce atmospheric interference. New radio telescopes are being built in radio quiet areas away from any people. Methods to adjust for the atmosphere on ground-based telescopes include complex adaptive optics and laser guides: shining lasers skywards to measure the atmosphere and adjust the optics to compensate.

Figure 6.1: The Planck satellite surveys the Cosmic Microwave Background (Artist’s concept). Credit: ESA/NASA/JPL-Caltech.

Gamma Rays

Gamma rays were discovered in 1900 and the first telescopes were in the 1960s. In cosmology, gamma rays are used to hunt for dark matter from possible annihilations of dark matter particles giving off gamma rays. Gamma rays are light that have the shortest wavelength, and the highest energy, and are produced in radioactive decay of elements. Their wavelength is so short they can pass through atoms. They are also produced by cosmic rays (which are extremely fast moving particles from space) when they hit the atmosphere. Most gamma rays from space do not get through Earth’s atmosphere so gamma ray astronomy is often based in space or balloons, although future detectors are likely to be on Earth to detect the most energetic gamma rays that interact in the atmosphere.

Gamma rays from space come from extreme environments such as emissions from supernova explosions and when black holes destroy stars at the centre of galaxies (active galactic nuclei). There are also gamma-ray bursts which are the most intense source of light known, lasting from 1 to 60 seconds. It. is not known what causes these bursts, although it could possibly be when neutron stars and black holes merge or when a black hole is born following a supernova. One gamma-ray burst can contain the same energy as all the energy the Sun will give out in it’s whole lifetime. There are various gamma-ray satellites in space: ESA’s INTEGRAL, NASA’s SWIFT and Fermi GST, the Italian AGIE and the Japanese GAP.

X-rays

X-rays have energies less than gamma rays, were discovered in 1895 and first used in astronomy in the 1950s. In space, they are produced in extremely hot gases and are used to study the remains of supernova explosions (particularly relevant for dark energy studies) and the hot gas in galaxy clusters that provides evidence for cluster dynamics and cosmic web formation. X-rays do not travel through the atmosphere so are detected from space or balloons. The Sun is a source of X-rays.

The NASA Chandra X-ray Observatory and ESA XMM-Newton satellite were both launched in 1999 and have been significant in providing X-ray survey data, including identifying black holes and galaxy clusters. Future X- ray satellites will look at magnetic fields in black holes, supernova and active galactic nuclei (NASA IXPE 2022), look at structure formation of the universe and dark matter (Japanese XRISM 2022) and map hot gas and give us more clues about the cosmic web (Europe/USA/Japan ATHENA 2031).

UV, Visible and Infrared Light

UV light was discovered in 1801 and has less energy than X-rays but more than visible. The universe viewed in UV looks different than in visible. The first astronomical UV detection was from the Sun in 1946 using a rocket-borne camera. UV light is given off by hot, young stars and dying stars that are growing hotter so we can use it to see where stars are in their evolution and to give us information on galaxy formation and evolution. UV light interacts with molecules, as we know when we get sunburn, and most of it is absorbed by the atmosphere so space or balloon based telescopes are required.

Visible light is a very narrow range of wavelengths and is the highest energy of light that can get through the atmosphere, that is why our eyes have evolved to see this light. Many telescopes use visible light and, since it is a narrow range, they often include infrared light as well.

Infrared (IR) light has lower energy than visible and, although it was discovered in 1800, it wasn’t until the 1960s that IR was used seriously in astronomy. Visible light is blocked by dust in space but IR can see through the dust to objects that visible cannot. IR telescopes can see much further distances than visible and the VLT (very large telescope) has seen a galaxy that existed when the universe was just 470 million years old. The VLT could see both headlights of a car on the moon.

The same mirrors and lenses can be used for IR and visible light so they can be designed to do both. IR telescopes need to be kept cool (with liquid nitrogen) to reduce the background thermal IR that the telescopes themselves produce. Water vapour in the atmosphere absorbs specific wavelengths of IR, and emits IR. so the telescopes tend to be in high, dry places, such as the VLT in the Atacama desert in Chile and the GTC (Gran Telescopio Canarias) in the Canary Islands. Above the atmosphere are the Spitzer Space Telescope, the Herschel Space Observatory and SOFIA (Stratospheric Observatory for Infrared Astronomy) that is flown in a plane.

The Hubble Space Telescope (IIST) was launched in 1990 and detects UV, visible and infrared light. It is one of the largest and most well known telescopes of recent times and has produced amazing images showing the wonder of the universe, as well as vast quantities of data that has been used for important science. HST has given us some of the most detailed images ever taken of galaxies and has also identified many new supernova that are used to measure the Hubble constant and dark energy.

The Dark Energy Survey (DES) has measured 300 million galaxies and, by measuring the distance to each galaxy, it gives clues on the evolution of galaxies, the cosmic web, dark energy and expansion and, as we look back far enough, it gives information on when the first stars formed (the Epoch of Reionisation).

Future telescopes are being planned that can look back even further, detect even more objects and look at even more detail of galaxies, clusters and supernova. Some of the planned satellite IR/visible telescopes are the NASA James Web Space Telescope in 2021, the ESA Euclid in 2022 and the NASA Nancy Grace Roman Space Telescope in 2025. Two large ground-based telescopes that are planned are the Vera Rubin Telescope and the ELT (Extremely Large Telescope).

Radio Waves

A radio wave is light that has a wavelength longer than infrared, starting at 1 millimetre and going to beyond 10,000 kilometres. Radio waves were predicted by James Clerk Maxwell in 1867 and radio astronomy started in 1931 when Karl Jansky discovered radio waves coming from the Milky Way. His name is used in the units measuring the strength of radio sources in astronomy.

Radio waves detect objects that are too cold to produce visible light and are used to detect neutral hydrogen atoms. This allows astronomers to study the cold clouds in interstellar space, distant galaxies back to when stars and galaxies first formed (Epoch of Reionisation) and molecular clouds where stars are born. Another source of radio waves comes from electrons moving in magnetic fields, called synchrotron radiation, that can be used to detect supernova remnants, pulsars and active galactic nuclei formed by black holes destroying the surrounding matter. Radio astronomy sees a lot of structure in galaxies that is not seen at other wavelengths.

Radio waves up to about 10-metre wavelength are not absorbed by the atmosphere and ground-based radio telescopes can be used. Telescopes need

Figure 6.2: SKA antennae. Artist’s impression of the dishes that will make up the Square Kilometre Array to be placed in South Africa. They will form a radio interferometer, with another set of antennae set up in Australia. Credit: SKA Organisation/Swinburne Astronomy Productions.

to be bigger than the size of the wavelength of light being measured, so radio telescopes need to be very large. Traditionally, they have been dishes (like radars) and the largest single dish radio telescopes are the 500 m FAST (Five- Hundred Metre Aperture Spherical Telescope) in China (2016), the 305 m Arecibo Observatory in Puerto Rico (1963) and the 100 m Green Bank Telescope in USA (2002), the biggest fully steerable single dish radio telescope.

To improve the ability of radio telescopes to detect high resolution images a technique called radio interferometry was developed by Martin Ryle in 1946. Multiple dishes (or detectors) are spread out and linked together and the image can be recreated from the interference patterns that are formed. The size of the telescope becomes the largest distance between two antennas in the array. This also increases the amount of signal collected by the number of detectors used. Figure 6.2 shows a future radio interferometer (due 2025), the SKA (Square Kilometre Array), based in South Africa, with another set of antennas in Australia. Existing radio interferometers are ALMA (Atacama Large Millimeter Array) with 66 antennas spread over a 16 kilometre distance in Chile, ASKAP and MEERKAT (as part of the SKA project), and the

European LOFAR (Low-Frequency Array) with 20,000 antennae spread across Europe. Simultaneous observations from a satellite and ground based telescope can make a very long baseline telescope the size of the separation between them.

Microwaves and the CMB

Microwaves are a subset of radio waves with wavelengths between 1 millimetre and 1 metre. They play an important role in astronomy because it is the region that the Cosmic Microwave Background is seen (the CMB is brightest at 2 millimetres). There have been many telescopes that have surveyed the sky for the CMB. the key ones are the satellites СОВЕ 1989-1993, IVMAP 2001-2010 and Planck 2009-2013. These telescopes have measured the CMB temperature, the blackbody spectrum, the temperature fluctuations (anisotropies), the CMB gravitational lensing and the Sunyaev-Zehdovich effect which can be used to identify distant galaxy clusters. From these CMB measurements the cosmological parameters have been determined.

Future CMB telescopes will be looking to improve the accuracy of these surveys and to look for polarisation modes of the CMB that may give clues for inflation. The next generation of CMB telescopes are called the CMB-S4 (Stage 4) and will be ground-based at the South Pole and the Atacama plateau in Chile planned for first light in 2029. The aim of CMB-S4 is to provide evidence for inflation by detecting the signature of primordial gravitational waves, to map dark matter in the universe, to determine neutrino mass, to look for limits on dark matter particles (axions), to improve dark energy measurements, and to test general relativity on large scales.

Computing

There is only one universe. We observe it’s behaviour to work out how it works but we cannot experiment with other universes or observe another universe. That is until the invention of the modern computer. Cosmologists can now gather evidence through experimentation using computer simulations where alternative universes can be created to test theories and ideas and see if they match what is observed. This has opened up a whole new way of doing astronomy.

This is where we can improve our precision cosmology'. With computers we can test out different models, create other universes and do experiments on them. This was never possible before, we had to observe what was there and make inferences. Now, we can compare what we see to other artificial universes. Such simulations need a lot of computing power and some require the use of supercomputers. Some of the significant computer cosmology simulations that have been produced are the Horizon Simulation, the Illustris TNG Project and the Millennium Simulation. Videos giving unique insights into the evolution of the universe are shown on their websites.

Astronomers also need computers to work the telescopes. Computers are used to clean the data from the telescopes, remove the atmospheric interference and any systematic issues from the telescope itself that need to be removed from the data. Images need to be cleaned and then they need to be catalogued. Computers can do this work but they are not as efficient as the human eye at identifying images. The general public are enlisted to help identify and catalogue images in citizen science projects at the Zooniverse website. Citizen science has become an important part of cataloguing images in surveys.

Current telescopes create so much data that astronomers cannot analyse it all, so we pick the data we want to use or have time to stud)'. Future telescopes that are being developed will create even larger amounts of data: terabytes of data a second. The only way to analyse this data will be to use computers. For this we need ever more sophisticated methods; machine learning and artificial intelligence are some of the approaches being investigated to handle such large amounts of data. This is new technology and is a fusion of computing and astronomy that could lead to exciting results in both fields.

Experiments

Cosmology covers many aspects of physics and requires bringing in evidence from experiments that are not cosmology based but have an impact on how we view the universe. In the mid-20th century, experiments in nuclear physics provided the evidence for the Big Bang Nucleosynthesis model that explains how the elements formed in the early universe and also how the nuclear processes in stars work. Today, cosmology has led us into the realm of quantum theory and the experiments happening in this field are providing us with evidence that could answer some of the outstanding Cosmological Problems. These experiments use high energy particle accelerators in places such as CERN and Fermilab (see Figure 6.3).

Then there are the experiments that are designed to detect dark matter particles that use particle accelerators and include using Earth itself as a particle detector in projects such as IceCube. Using cosmic rays as a source of high energy particles from the sky is another type of experiment that provides clues for cosmology; it was using cosmic rays that the first antimatter particle, the positron, was discovered. There is a particle detector on the International Space Station (AMS-02) that can detect cosmic rays.

There are many other experiments that are investigating the complexity of quantum behaviour, some of which may shed light on what happened in the early universe. These experiments will not be discussed here but for cosmolo- gists it is important that we know what clues these experiments are uncovering and, where appropriate, to apply the results to how the universe works. The overlap between astronomy and particle physics is the field of astroparticle physics and is now accepted as a distinct discipline in science.

Figure 6.3: ATLAS experiment at the Large Hadron Collider (LHC) CERN that is used to search for dark matter. Credit: ATLAS Experiment copyright 2020 CERN.