Talk: Why sometimes less is more in Cosmology

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Remote Talk: “Cosmology and fundamental physics with one-point statistics” by Cora Uhlemann for the Astro Seminar Series at the Waterloo Centre for Astrophysics (Canada) on Wed, Nov 11, 2020

Over the last decade cosmology has developed into a precision science that is able to determine key properties of our Universe by combining observations and theoretical models. So far a lot of this information comes from the Cosmic Microwave Background (CMB) that provides a snapshot of our Universe when it was only about 380,000 years old. The late-time large-scale structure (LSS) mapped by current and future galaxy surveys can in principle extract much more information by recording a motion picture of how structures form over time.

At early times the physics behind the formation of cosmic structures seen in the CMB is simple and linear. In particular, the distribution of matter is almost Gaussian, meaning that when one picks a random location, one is equally likely to find a region that is slightly more or slightly less dense than the average. A Gaussian distribution of matter is fully characterised by two-point statistics and has a “boring” one-point statistics.

The nonlinear clustering of matter over time changes this picture, because dense regions undergo gravitational collapse that makes them shrink and gain mass at the same time. This leaves a skewed distribution of matter at late times, which is non-Gaussian and has an interesting one-point statistics. We can extract additional cosmological information from this shape to complement two-point statistics and better pin down cosmological parameters.

The matter distribution at high redshift z (early times) almost Gaussian and symmetric around the mean density (rho=1). Gravitational clustering lets initially overdense regions (rho>1) collapse making them smaller and denser. At lower redshift (late times) most of the regions in the universe are underdense.

Talk: Lessons from timing the triple system

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This was a lightning talk I gave at the NANOGrav Fall 2020 meeting. Our project to test the Strong Equivalence Principle with PSR J0337+1715 did precision pulsar timing using the same telescopes in the same modes as NANOGrav but we found we needed to analyze the observations differently. So this is a lightning talk to point out some of the things we did differently and spark discussion about why. Most notably we found the polarization calibration procedure inadequate and so we implemented a procedure that fits for the polarization calibration simultaneously with fitting for the pulse arrival time; as a happy side-effect, polarization structure in the pulsar signal helps constrain pulse arrival times. You can read the slides, or the video is below (only the five minutes starting at 0:17 is my talk):

Event: LGBTQ+ in STEM day

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Wednesday November 18th is LBGTQ+ in STEM day, a day to celebrate the diversity of people who contribute to science, technology, engineering, and mathematics. The date represents American astronomer and gay activist Frank Kameny’s Supreme Court fight against workplace discrimination. For more information see https://prideinstem.org/lgbtstemday/ .

#LGBTSTEMDAY 18 November 2020 #LGBTQSTEMDAY
LGBTQ+STEM Day on social media

Here at Newcastle, this falls in our undergraduate “buffer week”, a short breather between classes. We would therefore like to invite students (PGR and undergraduate, LGBTQ+ and allies) to an online social get-together at 12:00; the zoom details were sent by email, contact us if you’d like to be included. We will suggest a few topics for discussion and/or a few social games, but please feel free to have lunch or a snack handy, and we will break into smaller groups for conversation.

A few topics I’d be happy to hear discussion on:

  • What can Newcastle and our School do to better support LGBTQ+ people?
  • How is the pandemic difficult for LGBTQ+ people in particular?
  • How can we build supportive communities under these conditions?

I would also like to draw your attention to a few resources that may be of interest:

I know that this list is somewhat US-centric, and also centred around physics and astronomy; I welcome suggestions to broaden its scope.

Anne Archibald (she/her) and Danielle Leonard (she/her)

Outreach: Creation of blind-accessible planetarium show

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Chris has been awarded public engagement grants from STFC and the Royal Astronomical Society to make a planetarium show and educational resources that are accessible to blind and vision impaired children. This build on a pilot project done last year at the British Science Festival in collaboration with blind astronomer Dr Nic Bonne. Uniquely, the planetarium show will use sounds and narration as the main methods of communication, with visuals acting as a secondary mode of communication. When COVID-19 restrictions are relaxed we will regularly visit our partner schools to also co-develop BVI-accessible classroom based activities around the subject of astronomy that use both sounds and tactile models. Here you can listen to a little taster of the soundtrack below and see a photograph of some of the tactile models.

A montage of astronomical pictures and photographs of the 3D "tactile" versions of these created with a 3D printer.
The 3D models showcased in this montage were created by Nicolas Bonne for A Dark Tour of the Universe, an astronomy show for the blind and visually impaired. Image Credit: ESO/M. Zamani, Nicolas Bonne, S. Brunier, TRAPPIST/ E. Jehin, EHT Collaboration, Sloan Digital Sky Survey, Millennium Simulation Project, NASA/ Goddard/ SDO, WMAP Science Team

Paper: Polarization details shed light on the origin of fast radio bursts

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Microsecond polarimetry of the repeating FRB 20180916B by K. Nimmo, J. W. T. Hessels, A. Keimpema, A. M. Archibald, J. M. Cordes, R. Karuppusamy, F. Kirsten, D. Z. Li, B. Marcote, and Z. Paragi

Fast Radio Bursts are millisecond-long incredibly bright radio flashes from distant galaxies. They are clearly produced by some kind of coherent emission process but the details or even basic nature of this process remain mysterious. Observational clues as to what it might be are few and far between, but polarization can provide hints about the geometry and magnetic fields in or near the emitting region. For this paper we managed to find polarization structure at very short timescales inside the burst – this is a clear sign that relatively small structures are producing elements of the radio burst. We still do not know what kind of structures these might be, but the Crab pulsar produces “giant pulses”, albeit still many orders of magnitude fainter than fast radio bursts, from coherent emission regions about 30 cm across. So this hint at detailed small-scale structure in fast radio bursts sheds a little light on their still-mysterious origin.

Plot showing bursts and their polarization.
Figure 4: detailed polarization structure of different sub-bursts. Note the microsecond time-scale on the bottom panel. Black bars indicate total intensity, red indicates linearly polarized intensity, and blue circularly polarized intensity (with sign). Note that these sub-bursts are almost completely linearly polarized. The grey bars in the top panels indicate the direction of linear polarization, with fuzziness indicating uncertainty. Note that there appears to be structured variation in the direction on microsecond timescales in the sub-burst shown in the bottom panel.

Paper: How do galaxies grow? Resolving gas-phase metallicity and star-formation at cosmic noon

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The Evolution of Gas-Phase Metallicity and Resolved Abundances in Star-forming Galaxies at z ~ 0.6 – 1.8 by S. Gillman, A. L. Tiley, A. M. Swinbank, U. Dudzeviciute, R. M. Sharples, Ian Smail, C. M. Harrison, Andrew J. Bunker, Georgios E. Magdis, J. Trevor Mendel and John P. Stott, accepted October 2020

The technique of integral field spectroscopy allows us to spatially-resolve the gas properties of individual galaxies. This is because we get a spectrum at every spatial pixel of the galaxy. By measuring the abundance of heavy elements (e.g., Nitrogen) compared to Hydrogen, the so-called “metallicity” we can learn about the star formation processes of the galaxies. Star formation inside a galaxy will enrich the gas with heavy elements (e.g., through supernovae and stellar winds) and will increase the “metallicity”. Strong star formation driven winds and supernova will distribute metals across the galaxy. In contrast, “pristine” material (mostly hydrogen) can also be accreted onto the galaxy through gas inflows. Consequently a lot can be learn by measuring the metallicity gradient inside individual galaxies we can learn about the distribution of star formation and the relevance of gas inflows.

Newcastle’s Chris Harrison is part of a team using extensive integral field spectroscopy from KMOS to study galaxies at the “cosmic noon” i.e., redshift z~1-2 when cosmic star formation was at its highest levels (KROSS, KGES and KURVS surveys). This paper uses these data to measure the galaxy-wide metallicity and metallicity gradients of ~650 star-forming galaxies at z~0.6 – 1.8. We find that for a given stellar mass, more highly star-forming, larger and irregular galaxies have lower gas-phase metallicities, which may be attributable to their lower surface mass densities and the higher gas fractions of irregular systems. Galaxies in our sample exhibit flatter metallicity gradients than local star-forming galaxies, in agreement with numerical models in which stellar feedback plays a crucial role in redistributing metals.

Metallicity gradients – i.e., the spatial gradient of the abundance of heavy elements across each galaxy – as a function of their redshifts (or equivalently, their cosmic time). There is no significant evolution between our two high redshift samples (blue and green circles); however, both samples exhibit slightly flatter gradients than observed locally (square data points). We also show theoretical predictions from two models of disc galaxies from Mott et al. (2013) with radially constant star-formation efficiency (purple dashed line) and variable star formation efficiencies (solid purple line).

Welcome

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We are launching our new research group website! We are a new group, interested in astronomy/astrophysics research topics mostly from an observational perspective. Our group properly started in 2020 with our first intake of PhD students and MRes students.

In this news feed we will post updates on our: research activities (papers, conferences, talks etc.), group membership and outreach/engagement activities.