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3D map of galaxies reveals treasures of the Cosmos
Team: Shadab Alam with eBOSS collaborationDate: 17 July 2020
The Sloan Digital Sky Survey (SDSS) have released a comprehensive analysis of the largest three-dimensional galaxy map of the Universe ever created, filling in the most significant gaps in our exploration of its history over 11 billion years of cosmic time.
The new results are detailed measurements of more than two million galaxies and quasars, derived from a subset of the SDSS: the extended Baryon Oscillation Spectroscopic Survey (eBOSS), which involved an international collaboration of more than 100 astrophysicists.
The detailed analysis of this dataset is described in more that 20 technical papers which the eBOSS team have made public. These papers, more than 500 pages in total, mark the completion of the key goals of the survey. Within the eBOSS team, individual groups at universities around the world focused on different aspects of the analysis. To create the part of the map dating back six billion years, the team used luminous red galaxies. Farther out, they used younger blue galaxies. Finally, to map the Universe eleven billion years in the past and more, they used quasars, which are bright galaxies lit up by material falling onto a central supermassive black hole. Each of these samples required careful analysis in order to remove contaminants and reveal the patterns of the Universe.
The team from the Institute of Astronomy at the School of Physics and Astronomy, including Dr Shadab Alam and Prof John Peacock, led an analysis focused on understanding the young blue galaxies. There is a long-standing question of nature vs nurture when one looks at the populations of different types of galaxies. More precisely, what aspects of the galaxy properties are affected by the local conditions around these galaxies? Such questions are interesting in their own right, but they are also particularly important to make sure our measurements of the properties of the Universe are not biased by local conditions of these galaxies.
The wealth of data released by the eBOSS team will continue to be one of richest datasets for astronomers to attack some of the most challenging questions in astrophysics.
The effort from the University of Edinburgh was supported by the European Research Council through the COSFORM Research Grant.
Deflections of remnant light from the big bang through cosmic filaments
Team: Siyu He, Shadab Alam , Simone Ferraro, Yen-Chi Chen and Shirley HoDate: 12 April 2018
The deflection of starlight through the sun during the solar eclipse in 1919 showed the first success of Einstein's General Theory of Relativity proposed in 1915 and made him most famous scientist. Since then the idea of gravitational lensing, bending of light due to the influence of mass, has been developed to provide some of the most precise measurements of our Universe. The gravitational lensing also provides one of the most convincing evidence of dark matter envelope around galaxies known as dark matter halos. These dark matters halos introduce an extremely small deflection in the light and hence distorts the image of background galaxies resulting in a circle to appear slightly elliptical to us. Such effect is extremely small and impossible to measure for a single galaxy, therefore, requires averaging them for millions of galaxies to detect any measurable signal called "Weak gravitational lensing". This measurement leads to an understanding of how much galaxies grows with time by eating more and more of matter around them. They also give us the ability to study how a light beam reacts to mass around it over cosmic time.
Another celebrated observation in the study of the universe is called Cosmic Microwave Background (CMB), the remaining light after the big bang. The accidental discovery of CMB by Penzias and Wilson in 1964 led to the noble prize in 1978. The CMB has been at the forefront of cosmological information since its first discovery and one of the most prominent evidence confirming GR. What we see in this remnant light from the big bang is that it is almost same in all direction with a tiny fluctuation in its brightness at the level of one part in million. Now the fluctuation themselves have a certain shape. When this remnant light travels from the early universe to us it passes through millions of galaxies and dark matter halos on the way which introduces tiny deflections due to weak gravitational lensing and has been measured with very high signal to noise.
In principle, the deflection of light from CMB can be caused by all the matters in the universe and not necessarily just around galaxies. But since galaxies live in the densest region of the Universe the signal of such deflection will be strongest from those parts. In the standard model of the universe, the dark matter is not only in the halos around galaxies but are also spread in line connecting galaxies known as filaments. Therefore such filaments should also cause deflection of CMB light and hence will produce measurable weak gravitational lensing signal. Recently our team have detected such a signal for the first time. This is the first step in going beyond the standard weak lensing of CMB around galaxies and should lead to an improved understanding of central problems of cosmology like dark matter and dark energy. This could have great potential to also test whether Einstein is correct in an environment around filaments where many models will show a different behaviour even if they behave same around galaxies.
You can find the technical paper describing the study over here
This was in news over here and here
A new test of Einstein's General Theory of Relativity
Team: Shadab Alam , Hongyu Zhu, Elena Giusarma, Rupert Croft and Shirley HoDate: 21 September 2017
Description: In 1960, two Physicists, one on the roof of the Harvard Physics Laboratory, and one in the basement carried out a major test of General Relativity (GR), Einsteins theory of gravity. Robert Pound and Glen Rebka were looking for evidence of a gravitational redshift of light. GR predicts that photons climbing up out of the Earth's gravitational field will lose energy and therefore increase in wavelength. The measurement was successful and it became one of the four experimental pillars establishing the validity of GR on Earth and Solar System scales.
If we fast-forward to the present day, we once again find ourselves in need of tests of our theories of gravity, but this time on much larger scales. The Universe has been observed to be accelerating in its expansion (the research was awarded the Nobel Prize in 2011). However, a simple accounting of its contents leads to the expectation that it should be decelerating. There are two leading possibilities to explain this. Either a previously unknown substance with negative pressure (``Dark Energy'') dominates the energy balance of the Universe and causes it to accelerate (while at the same time being very smooth and therefore unnoticeable on Solar System scales). The other is that our theory of gravity, GR, while working to high precision on small scales, somehow breaks down when we approach intergalactic and cosmological scales, and so does not cause the attraction which would make the Universe decelerate. In order to tell these possibilities apart, we, therefore, need to extend our tests of gravity to these scales, millions of light years in size. Measuring gravitational redshifts of entire galaxies offers a new way to do this.
Our team of associated with Institute for Astronomy of the University of Edinburgh and Carnegie Mellon Physics Department recently made the first detection of such galaxy gravitational redshifts by analyzing half a million galaxies taken from the Sloan Digital Sky Survey. We found when comparing massive galaxies to less massive ones the more massive ones had a greater redshift on average, as expected in Einstein's theory. They have also developed a new framework to model these intricate effects in simulations. This measurement opens up a new window on General Relativity and alternatives. The result appears as a series of four papers in the Monthly Notices of the Royal Astronomical Society and paves the way for more precise tests with future data.
A video summarizing the contents of the papers for a technical audience appears on the CMU Cosmology Youtube Channel.
An introduction to gravitational redshifts for the interested public is also posted on Youtube Channel
The series of relevant four paper are as follows:
1) N-body Simulations
2) Theoretical Approach
3) Relativistic Beaming
4) Measurement of GZ
This was in news over here
https://www.cmu.edu/physics/news-events/news-archive/2017/croft-general-relativity.html