UNDERSTANDING DARK ENERGY

To find out more about dark energy' we want to measure how the universe has expanded over time. From this we can determine it’s strength and whether it has changed, and maybe get some clues as to what dark energy is. We explore the history of expansion by looking back in time at the most distant galaxies and at the CMB, the furthest point we can see. The techniques to measure the expansion history are mostly the same as the techniques we use to measure the Hubble constant and having a more accurate Ho value will improve our dark energy measurements as we compare the value of expansion today with that of the past.

The main difficulty with understanding dark energy is that we cannot see it directly, we measure it’s effects on other things, mainly the positions of stars (from supernova), galaxies (from the BAOs) and the structure within the cosmic web (from the shape of voids). What complicates the picture is that there are other processes that also affect galaxy distribution and the cosmic web and we have to unpick the dark energy effects from the others.

BAOs are a standard ruler within the universe and the BAOs in the distribution of increasingly distant galaxies can give us one of the most reliable measurements of how dark energy has affected the expansion history. To make these measurements we need to know the distance of millions of galaxies. The Dark Energy Survey (DES) measured the distance to 300 million galaxies going back as far as 9 billion years. It also detected 200,000 galaxy clusters for cluster counts and 207 supernova to use as standard candles.

Dark energy can also be detected using voids in the cosmic web. Voids are cosmic bubbles in the universe, regions where there are very few galaxies, and the effect of expansion on voids is relatively simple (compared to dense regions). By looking at how the shape, size and number of voids change with time it is possible to gather information about the expansion of the universe as well as other cosmological parameters. Surveys of large areas of the sky are needed to map the voids.

What the results are uncovering is a discrepancy between the dark energy measurements from the CMB and those from supernova. As with the Hubble tension there seems to be a difference between the early universe measurements and the late universe measurements. The difference is not as much as for the Hubble tension but it is enough for scientists to want to know what is causing it. Current and future surveys (the Vera Rubin, LOFAR, SKA and Euclid) will detect even larger numbers of galaxies, out to even further distances, from which we can test the discrepancy, test quintessence and other dark energy models.

HOW DO GALAXIES AFFECT COSMOLOGY?

In cosmology, we treat the universe as one system where there are particles that gravitate together within an expanding space. On the large scale and in the early universe this works very well. On a scale the size of galaxies there are more complex processes happening that affect the way the universe behaves and complicates our cosmology models; the formation and evolution of galaxies are more difficult to model and predict. As the accuracy of cosmology has improved the effects of galaxy evolution has become more important.

We would like to understand what these effects are and how they influence cosmology. To do this we need to understand more about galaxy evolution.

The way galaxy evolution affects how the structure of matter evolves has to be taken into account when making cosmology measurements. As structure grows the baryonic acoustic oscillations (BAOs) in the distribution of galaxies are stretched and affect measurements of expansion, dark energy and cosmological parameters. Galaxies tend to grow where the dark matter is, we call this ‘bias’, and also has to be taken into account when measuring BAOs. We would like to know more about the bias between where galaxies and dark matter exist.

As we look back in time there is a period when star formation was at it’s peak when the universe was about 3.5 billion years old. This period is known as the cosmic noon. The universe was three times smaller and 27 times denser than it is now and galaxies were closer together. Images from the Hubble Space Telescope show that galaxies were smaller and had clumpy, irregular shapes. Today, galaxies are bigger and have regular, smooth shapes. A prediction of the Big Bang is that the universe would have looked different in the past and these IIST observations confirm this prediction providing another Cosmological Clue.

The universe was also much brighter at cosmic noon. Theory says that stars were much bigger and shone more brightly than the lower mass stars of today. Stars prefer to form in spiral type galaxies and yet most of the galaxies today are elliptical shaped (spheroids). How and when did this change happen? We would like to know more about what the galaxies looked like then and how they evolved into the galaxies of today. Since galaxies were closer together mergers would have been more likely to occur and this is one way that bigger, smoother galaxies could have formed, although, evidence from computer simulations shows that the processes are more complex than this.

Going further back in time we come to the cosmic dawn, this is the name given to when stars first started to form at about 150-400 million years from the Big Bang. When the first stars formed they were massive stars that gave off a lot of strong UV light. This light was absorbed by the neutral hydrogen atoms, releasing electrons and creating ionised hydrogen. A bubble of ionised hydrogen formed around the stars and over a period of about a billion years these bubbles grew and merged until the universe was fully ionised and became transparent to light, which is why we can see the stars and galaxies today. This process is called reionisation and the time it took from the first ionised bubbles to all the universe being ionised is called the ‘Epoch of Reionisation’ (EoR). We cannot see stars this far back but there are other ways we can detect when the cosmic dawn happened using the CMB. Light at specific wavelengths are missing from the CMB and this can be used to determine when reionisation started.

W 'e are able to see galaxies within the Epoch of Reionisation; deep galaxy surveys can detect galaxies when the universe was less than a few billion years old. The most distant galaxy observed by IIST is when the universe was 400

million years old. The observation of this galaxy has caused a puzzle because theory predicts that galaxies should form later than this. Maybe there is some physics happening that has not been included in our models. As we look back further and further we can study the detail of what happened. Information from the CMB is just a snapshot in cosmic history. By studying galaxies through the whole epoch we can get a more detailed three-dimensional picture of how the primordial fluctuations evolved into the cosmic web.

The early phase of galaxy formation is not well understood. Protogalaxies are the precursors to galaxies. Identifying protogalaxies is not easy. They formed a long time ago and are very large, diffuse structures and we have questions about them that could help us understand the formation of structure. Did these protogalaxies form from dark matter collapsing with stars and black holes forming all at the same time, or was it more complex than this? Did dark matter halos merge together before stars could form, did protogalaxies exist before the stars formed, and what role did star formation have in developing the galaxy structures?

With future radio telescopes it may be possible to see even further back into the dark ages before the first stars formed. There is one specific wavelength of light at 21 centimetres that is emitted by neutral hydrogen. During the dark ages this light will have been emitted. It will be a tiny signal but if it can be detected then it may tell us something about the environment that existed before the first stars formed.

There is much to learn about how the evolution of stars and galaxies affects the universe, both in the early years and their continuing influence today. We still have many questions about the early formation of structure in the universe and studying distant galaxies could provide some answers to these questions. Finding such distant galaxies and understanding the dark ages, cosmic dawn and cosmic noon requires extremely sensitive telescopes and clever use of the light that interacts with atoms. Searching in the early universe uses all the skills of astronomers and the results will provide essential clues on how stars and galaxies influenced the evolution of the universe.

IMPROVING THE COSMOLOGICAL PARAMETERS

The combination of theoretical developments in the ACDM model and wide- ranging observational surveys has resulted in a set of cosmological parameters that define the ACDM model to a high degree of accuracy; this is precision cosmology. Most of the ten (and more) cosmological parameters that define the model are known to a level of 1% or better and we would like to improve this. There are some parameters where it is difficult to separate the values, they depend on each other, so we would like more evidence to enable us to separate them out.

The most powerful source for defining the cosmological parameters comes from CMB data. The set of parameters from the Planck team in 2018 is the current benchmark. These combine the CMB data with the data from BAOs as a separate and complimentary measurement technique and together they improve the accuracy of the parameters.

The cosmological parameters cover all aspects of cosmology and improving them requires all the techniques that have been discussed in this chapter: dark matter particle experiments, dark energy supernova surveys, large area galaxy surveys to detect the expansion rate from BAOs, wide area surveys to explore the cosmic web voids, X-ray measurements of clusters, gravitational lensing detection of dark matter and computer modelling to pull all the information together into a parameter set. New experiments and survey methods will improve the accuracy of cosmological parameters and provide more stringent tests of the ACDM model.

There are many ways that ACDM is being tested, questions being answered and Cosmological Problems being solved. It is an interesting time to be an astronomer as new telescopes and experiments come online. Each new telescope, particle detector, experiment and computer simulation adds to our understanding of how the universe works. There is plenty of work to do to test theories, discoveries to make and a universe to explore.