Cosmology and Fundamental Physics from High Precision CMB Lensing Science

Lead Research Organisation: University of Cambridge
Department Name: Institute of Astronomy

Abstract

One of the most striking facts about our universe is that most of its contents are invisible. More than 80% of the matter in our universe is not made of atoms, but instead of invisible dark matter, distributed in an enormous filamentary dark matter network that underlies all visible objects. The form of this dark matter distribution encodes a wealth of information about the contents, origin, and evolution of our universe.

How can we see such invisible dark matter structures? Though dark matter does not emit or scatter light directly, it exerts a gravitational pull that allows us to observe its presence indirectly: a clump of dark matter gravitationally attracts rays of light that are passing by, deflecting their paths and causing everything that lies behind to appear magnified. Observation of this gravitational lensing effect allows us to infer the presence of dark matter. To map out the matter distribution, we can search for subtle lensing features in the most distant source of light: the afterglow of the hot big bang, the cosmic microwave background radiation (CMB). This CMB radiation has traversed the entire cosmic web of dark matter before reaching our telescopes. By finding lensing features in this CMB light, we can reconstruct maps of the matter distribution projected across the entire observable universe.

In past work, I made some of the first measurements of this CMB lensing effect. Now, for the first time, new experiments will provide CMB data of extremely high quality, which have immense potential for high-precision lensing measurements of the dark matter distribution. However, the lensing features are more than a hundred thousand times smaller than the mean brightness of the CMB, so measuring them reliably from noisy data can be challenging at this level of precision. My research program will involve work in theory, simulation, statistical methods and data analysis that will enable such powerful lensing measurements. Analyzing data from upcoming CMB surveys known as AdvancedACT and Simons Array, I will extract the lensing signal at unprecedented precision and construct a high-resolution map of the dark matter distribution across much of the universe.

Such highly precise CMB lensing mass maps will be powerful probes of new physics. For example, the form of the cosmic dark matter distribution is affected by the presence of neutrinos, a type of particle with poorly understood properties. Though neutrinos make up a quarter of the known elementary particles, their masses are completely unknown. The shape of the distribution of dark matter depends on the masses of these particles, because the more massive neutrinos are, the more their motions smooth out fine features in the cosmic dark matter distribution. With precise CMB lensing maps, I will measure the detailed shape of the matter distribution and hence determine how massive neutrinos are. This will elucidate the properties of this mysterious type of particle and give insight into the physical origin of their masses.

Precise knowledge of the CMB lensing signal will also allow us to learn more about the beginning of the universe. Our leading theory for the cosmic origin is inflation -- a mechanism that causes the universe to initially expand exponentially fast. However, this mechanism has not been definitively established, and little is known about the energy with which it took place. While a certain characteristic pattern (B-modes) in the polarization of the CMB would be definitive evidence for inflation and would determine its energy, measurements of this inflationary pattern are currently limited because inflationary effects can be confused with similar effects from lensing. However, if we can directly measure the CMB lensing signal, we can disentangle the lensing effects from the inflationary effects. With the precise lensing maps I construct, I will thus enable more powerful constraints on inflation and the beginning of our universe.

Publications

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Ade P (2019) The Simons Observatory: science goals and forecasts in Journal of Cosmology and Astroparticle Physics

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Aiola S (2020) The Atacama Cosmology Telescope: DR4 maps and cosmological parameters in Journal of Cosmology and Astroparticle Physics

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Baleato Lizancos A (2022) Delensing the CMB with the cosmic infrared background: the impact of foregrounds in Monthly Notices of the Royal Astronomical Society

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Barron D (2018) Optimization study for the experimental configuration of CMB-S4 in Journal of Cosmology and Astroparticle Physics

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Baxter E (2021) Determining the Hubble constant without the sound horizon scale: measurements from CMB lensing in Monthly Notices of the Royal Astronomical Society

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Baxter E (2019) Constraining the rotational kinematic Sunyaev-Zel'dovich effect in massive galaxy clusters in Journal of Cosmology and Astroparticle Physics

 
Description ERC Starting Grant
Amount € 1,492,065 (EUR)
Funding ID 851274 
Organisation European Research Council (ERC) 
Sector Public
Country Belgium
Start 01/2020 
End 12/2024
 
Description Simons Observatory 
Organisation Simons Observatory
Country Chile 
Sector Academic/University 
PI Contribution I have served as co-lead of the lensing working group. In the course of my work, we have performed key forecasts for inflationary gravitational wave constraints, neutrino masses, and dark energy that have allowed for an optimization of the experiment design.
Collaborator Contribution My collaboration partners have made an enormous amount of progress in completing a significant fraction of the experiment design, resulting in the start of construction.
Impact While it is still very early for Simons Observatory, we have begun construction of parts of the telescope following completion of much of the optics design. A paper summarizing our work thus far is in preparation.
Start Year 2017
 
Description Cambridge Philosophical Society Talk 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach Regional
Primary Audience Public/other audiences
Results and Impact I gave a major outreach talk about the CMB as part of the Cambridge Philosophical Society lecture series "What We Don't Know About the Universe". This was held as part of the Cambridge Science Festival and attracted a large audience of hundreds of people.
Year(s) Of Engagement Activity 2018
 
Description Discovery Channel Outreach Documentary Series Participation 
Form Of Engagement Activity A broadcast e.g. TV/radio/film/podcast (other than news/press)
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Public/other audiences
Results and Impact Helped develop and was interviewed for Discover Channel outreach video series on cosmology and gravity.
Year(s) Of Engagement Activity 2020
 
Description Global Cambridge London 
Form Of Engagement Activity A formal working group, expert panel or dialogue
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Supporters
Results and Impact I participated in a panel discussion, and gave a short talk, at a "Global Cambridge London" event discussing Stephen Hawking's Legacy.
Year(s) Of Engagement Activity 2018
URL http://www.ctc.cam.ac.uk/news/181218_newsitem.php
 
Description Large popular science talk at "Bay area beacon of science" event 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Public/other audiences
Results and Impact I gave an outreach talk / colloquium to a general audience at a regular astronomy event.
Year(s) Of Engagement Activity 2017