Marcelle Soares-Santos

‣
Submitted information from google form

Table of contents

Helpful webpages:

https://lsa.umich.edu/physics/people/faculty/marcelle-soares-santos.html

Slides PDF:

Speaker notes:

Introduction slide / General speaker notes

Researcher's background:

Soares-Santos was born and raised in Brazil, where she studied physics at the Federal University of Espírito Santos and received her bachelor’s degree in 2004. She completed her graduate studies in astronomy at the University of São Paulo, where she received her M.Sc. and Ph.D. in 2010. In the same year she moved to the U.S. and became a Postdoctoral Research Associate in Astrophysics at the Fermi National Accelerator Laboratory (Fermilab) in Batavia where she helped build the DEcam. She received the Alvin Tollestrup Award for her work at Fermilab in 2014. She then became an Assistant Professor of Physics at Brandeis University in 2017, was awarded a Sloan Research Fellowship in 2019, and has been an Assistant Professor at the University of Michigan at Ann Arbor since 2020. Her research was recently featured in the PBS documentary series “Nova Wonders”.

Synopsis of Work:

Soares-Santos works in multi-messenger astronomy to measure cosmic acceleration and understand the nature of dark energy. She contributed to the construction of the Dark Energy Camera (DEcam) for the Dark Energy Survey (DES). The DES is an international collaboration of researchers that aims to constrain the properties of dark energy using near-ultraviolet, visible, and near-infrared imaging of supernovae and galaxies. Using the DEcam, Soares-Santos and her collaborators were able to detect the electromagnetic counterpart to the gravitational wave event GW170817 due to the collision of two neutron stars in the galaxy NGC 4993.

Citations and resources:

https://lsa.umich.edu/physics/people/faculty/marcelle-soares-santos.html

https://www.darkenergysurvey.org/the-des-project/science/

https://en.wikipedia.org/wiki/Dark_Energy_Survey

https://en.wikipedia.org/wiki/Marcelle_Soares-Santos

https://astrobites.org/2021/01/09/meet-the-aas-keynote-speakers-dr-marcelle-soares-santos/

Societal relevance of work:

Dark energy is the name given to the unknown force that drives cosmic expansion. The first evidence of dark energy came from observations of distant supernovae which showed that the Universe does not expand at a constant rate. Supernovae are used as “standard candles”: astronomical objects that produce light of a known brightness as its source. Measuring the rate of cosmic expansion involves comparing the light’s observed brightness, known brightness at the source, and its recorded redshift. Recently, cosmologists have begun to use gravitational wave sources as “standard sirens” to directly determine the rate of cosmic expansion. Soares-Santos searches for gravitational wave-emitting events as well as employs traditional methods such as galaxy clusters and gravitational lensing to further the understanding of the accelerated expansion of the Universe.

Slide 1: Gravitational waves

Science details:

Accelerating masses cause gravitational waves which are ripples in spacetime. In 1916 Albert Einstein predicted the existence of gravitational waves in his general theory of relativity. The theory showed that massive accelerating objects (such as supernovae, or binary systems of stars and/or black holes) would cause waves of spacetime to propagate at the speed of light in all directions away from the source. These waves are invisible but squeeze and stretch spacetime as they pass by. In September 2015 LIGO made the first direct confirmation of the existence of gravitational waves. This was due to the collision of two black holes 1.3 billion light-years away - causing a ripple in spacetime on Earth that was 1000 times smaller than the nucleus of an atom. In October 2017, the LIGO and Virgo collaborations announced the first detection of gravitational waves originating from merging neutron stars in a binary star system.

In the 1990s, observations of supernovae were the first indicators of the existence of dark energy, which showed that the Universe’s expansion is accelerating - when it was previously thought that the expansion should decelerate over time. Dark energy describes the unknown force that causes this expansion. When the Universe expands, it stretches the wavelength of radiation (gravitational waves, electromagnetic radiation) causing it to shift toward the red end of the electromagnetic spectrum (redshift). Detections of gravitational waves can provide information about the current rate of expansion of the Universe and the composition of dark energy.

Citations and resources:

https://www.ligo.caltech.edu/page/what-are-gw

https://en.wikipedia.org/wiki/Gravitational_wave

https://www.news.ucsb.edu/2019/019393/standard-siren

https://science.nasa.gov/astrophysics/focus-areas/what-is-dark-energy

Figures:

Left: Cartoon depicting cosmological redshift. At an earlier time (left) a photon from a distant galaxy is seen on Earth with an original wavelength (yellow). At a later time (right), when the space between the Earth and the distant galaxy has expanded, the photon from the distant galaxy is seen on Earth with a stretched (redshifted) wavelength (red). https://www.thoughtco.com/what-is-redshift-3072290

Right: Animation of two neutron stars orbiting each other and visualization of the gravitational waves that propagate away from the source. https://www.insidescience.org/news/gravitational-waves-throw-light-neutron-star-mergers

Slide 2: The Hubble constant (I)

Science details:

When the Universe expands and celestial bodies are pulled away from us, the wavelength of light is shifted toward the red end of the electromagnetic spectrum (redshift). If the source of light is receding faster then the light will be more redshifted. The speed (v) of the object is related to its distance from us (d) by the Hubble constant (H₀): v = H₀d. To understand the accelerating expansion of the Universe, scientists have tried to accurately determine the Hubble constant by measuring the redshift of celestial objects and their distances from us.

This requires a standard candle: an object that always has the same standard brightness. Computing the distance to the object involves measuring how dim it appears on Earth compared to its known brightness at the source. Astronomers need to use a “distance ladder” to calibrate the distances to certain objects using previous measurements of distances to others, which can introduce uncertainties at each step of the ladder.

Cosmologists currently have two different values for the Hubble constant: one that was calculated using the cosmic microwave background (H₀~68), and one that uses Type 1a supernovae (H₀~73) - both of which have small error bars. This disagreement might mean that there is missing information in the understanding of the Universe’s expansion. Gravitational waves, however, can be used as a “standard siren” to determine the distance to an object and might yield a different value of the Hubble constant. Unlike standard candles, the standard siren does not rely on a distance ladder - the distances can be computed directly.

Citations and resources:

https://www.news.ucsb.edu/2019/019393/standard-siren

https://skyandtelescope.org/astronomy-news/tension-continues-hubble-constant/

Figures:

Depiction of the Three Steps to the Hubble constant. Cosmic structures at different distances from Earth from left to right: Cepheids within the Large Magellanic Cloud (180,000 light years), galaxies hosting Cepheids and Type 1a supernovae (24-100 million light years), and distant galaxies in the expanding Universe hosting Type 1a supernovae (100 million-1 billion light years). The light from the distant galaxies is redshifted (stretched by the expansion of space). https://theness.com/neurologicablog/index.php/mystery-of-the-hubble-constant/

Slide 3: The Hubble constant (II)

Science details:

When the Universe expands and celestial bodies are pulled away from us, the wavelength of light is shifted toward the red end of the electromagnetic spectrum (redshift). If the source of light is receding faster then the light will be more redshifted. The speed (v) of the object is related to its distance from us (d) by the Hubble constant (H₀): v = H₀d. To understand the accelerating expansion of the Universe, scientists have tried to accurately determine the Hubble constant by measuring the redshift of celestial objects and their distances from us.

This requires a standard candle: an object that always has the same standard brightness. Computing the distance to the object involves measuring how dim it appears on Earth compared to its known brightness at the source. Astronomers need to use a “distance ladder” to calibrate the distances to certain objects using previous measurements of distances to others, which can introduce uncertainties at each step of the ladder.

Cosmologists currently have two different values for the Hubble constant: one that was calculated using the cosmic microwave background (H₀~68), and one that uses Type 1a supernovae (H₀~73) - both of which have small error bars. This disagreement might mean that there is missing information in the understanding of the Universe’s expansion. Gravitational waves, however, can be used as a “standard siren” to determine the distance to an object and might yield a different value of the Hubble constant. Unlike standard candles, the standard siren does not rely on a distance ladder - the distances can be computed directly.

Citations and resources:

https://www.news.ucsb.edu/2019/019393/standard-siren

https://skyandtelescope.org/astronomy-news/tension-continues-hubble-constant/

Figures:

Left: The Hubble constant measured using different methods is plotted against time. The Cepheid method (blue) uses Type 1a supernovae in conjunction with Cepheid variable stars, which relies on observations of the current Universe and gives higher values of the Hubble constant. The cosmic microwave background (CMB) method (black) relies on observations of the early Universe and gives lower values. The “tip of the red giant branch” (TRGB) method (red) gives intermediate values. For all methods, the values become more precise and the discrepancy between the values grows over time. https://aasnova.org/2020/07/10/shining-bright-through-the-ages/

Right: Hubble’s Law (velocity = Hubble constant ✕ distance) is shown by plotting velocity against distance for a number of galaxies. Galaxies closest to us are moving away at a slower velocity.speed, and galaxies furthest from us are moving away at a faster velocity/speed. https://astrobites.org/2016/04/20/conflicts-between-expansion-history-of-the-local-and-distant-universe/

Slide 4: Multi-messenger astronomy

Science details:

There are four extrasolar “messengers” in astronomy: electromagnetic radiation (photons), gravitational waves, neutrinos, and cosmic rays. They are called messengers because each signal carries a message containing information about its source. The messengers are created through different astrophysical events such as solar flares, supernovae, gamma ray bursts, active galactic nuclei, relativistic jets, and neutron star mergers. Each type of event produces one or more messengers and gives insights into the processes that created them.

For example, a neutron star collision occurred in the galaxy NGC 4993 produced the gravitational wave signal GW170817 which was observed by the LIGO/Virgo collaboration, a gamma ray burst GRB 170817A which was observed by the Fermi Gamma-ray Space Telescope and INTEGRAL, and ultraviolet, X-ray, and radio signals observed by the Neil Gehrels Swift Observatory, Chandra X-ray Observatory, and Karl G. Jansky Very Large Array, respectively. These combined observations marked a new milestone for multi-messenger astronomy because they were the first detections of a gravitational wave event with an electromagnetic counterpart. Soares-Santos and her collaborators used the Dark Energy Camera (DEcam) to detect these electromagnetic counterparts to GW170817, along with 6 other teams from different observatories.

Citations and resources:

https://en.wikipedia.org/wiki/Multi-messenger_astronomy

https://chandra.harvard.edu/photo/2018/gw170817/

https://astrobites.org/2021/01/09/meet-the-aas-keynote-speakers-dr-marcelle-soares-santos/

https://news.umich.edu/ligo-virgo-make-first-detection-of-gravitational-waves-produced-by-colliding-neutron-stars/

Figures:

Left: Cartoon depicting the four messengers involved in multi-messenger astronomy. The multi-messenger source is a binary star system (white) which emits signals detected on Earth: gamma rays which are absorbed (blue arrows), neutrinos (red arrow), cosmic rays which undergo magnetic deflection (green arrow), and gravitational waves (yellow arrow). https://nbi.ku.dk/english/research/experimental-particle-physics/icecube/astroparticle-physics/

Right: Illustration of a neutron star merger. The stars are shown in blue and white, spacetime is illustrated by grid which is warped due to gravitational waves, which are illustrated in shades of purple. https://chandra.harvard.edu/photo/2018/gw170817/

Slide 5: The dark energy camera

Science details:

The Dark Energy Survey (DES) is an international collaboration of over 400 scientists from over institutions that aims to understand the properties of dark energy by doing astronomical surveys. Soares-Santos contributed to the construction of the Dark Energy Camera (DECam): a 570-Megapixel digital camera on the Blanco 4-meter telescope at Cerro Tololo Inter-American Observatory in the Chilean Andes that carries out projects for DES. DECam took “Official First Light” images in 2012, and has since been used to detect light from hundreds of millions of distant galaxies and thousands of supernovae (more than any other single astronomical survey). DES is also using weak gravitational lensing and galaxy clusters which, in combination with supernovae observations, allows DES to constrain changes in dark energy over the course of cosmic time.

Citations and resources:

https://en.wikipedia.org/wiki/Dark_Energy_Survey

https://www.darkenergysurvey.org/the-des-project/overview/

https://www.darkenergysurvey.org/the-des-project/science/

Figures:

Left: Dark Energy Survey logo. https://en.wikipedia.org/wiki/Dark_Energy_Survey#/media/File:Dark_Energy_Survey_logo.jpg

Right: Illustration of the timeline of the cosmos spanning the Big Bang Expansion (13.7 billion years). From left to right: quantum fluctuations, inflation, afterglow light pattern (400,000 years), dark ages, first stars (about 400 million years), development of galaxies, planets, etc., and dark energy accelerated expansion. https://www.darkenergysurvey.org/wp-content/uploads/2016/01/timeline.jpg

Slide 6: GW170817

Science details:

DEcam is able to scan an area of the sky quickly and with a wide field of view, making it ideal to look for electromagnetic counterparts to gravitational waves. LIGO is not able to pinpoint the exact location of gravitational wave sources, which makes DEcam especially useful. The original goal of DES was to search for supernovae, so Soares-Santos had to adapt DEcam to also search for gravitational wave EM counterparts. She and her collaborators were successful in detecting these EM counterparts to the gravitational wave event GW170817 (due to two neutron stars colliding over 100 million years ago) in 2017, along with 6 other teams from different observatories.

This detection will help astronomers better understand the dynamics of neutron stars, as well as how heavy elements (such as gold and silver) were created - which requires an understanding of the high-energy astronomical events that produce them. In the long term, these observations will contribute to furthering the understanding of the Universe’s accelerating expansion and uncovering the properties of dark energy.

Citations and resources:

https://www.brandeis.edu/now/2017/october/gravitational-SoaresSantos-waves.html

https://astrobites.org/2021/01/09/meet-the-aas-keynote-speakers-dr-marcelle-soares-santos/

Figures:

Left: DEcam imager with CCDs (blue) in place. The CCDs make up the main component of the imager and are arranged in a hexagonal pattern on the focal plane of DEcam. https://www.darkenergysurvey.org/wp-content/uploads/2015/04/11-0222-13D_hr2-682x1024.jpg

Right: Two optical images from the DEcam of GW170817 showing a transient source near the galaxy NGC 4993, its location is marked by the reticle. Its first detection occurred 0.5-1.5 days post merger (left) and is shown to have faded >14 days post merger (right). https://news.umich.edu/ligo-virgo-make-first-detection-of-gravitational-waves-produced-by-colliding-neutron-stars/

Initial Notes

November 2020 Lecture: “Multi-messenger cosmology with gravitational waves” https://www.youtube.com/watch?v=bysdFYNbx7E

  • Ligo and Virgo detected gravitational waves from GW170817 (1st neutron star merger observed). She found the first kilonova (?) and observed the evolving light curve.
image
  • Above image (from 2017?) is a comparison of “discovery image”: night of first neutron star merger vs. same region 2 weeks later. Main takeaway: source (optical counterpart of merger) is the little dot in cross-hairs. We want to identify a faint source (here 4 mm telescope, few seconds exposure) at 40 Mpc (close for cosmologists) but it’s challenging when they’re short-lived. Lots of potential science investigations from these events (collision of neutron stars), but the main motivation for her is cosmology.
image
  • Cosmology motivation: H_0 rate of expansion of universe today. Several precise measurements (supernova = SNe or local or CMB), why bother with new? Above represents the problem (few years old). Latest results from Planck using CMB. Results disagree significantly. Authors computed how much H_0 would move if you changed cosmological parameters (top arrows) - that would improve things. If you allowed something to change with time, also would improve. Neutrino numbers (sterile…) would make a change.
  • Her focus: also invest in a new observable: multi-messenger … merger … black holes.
  • Dark Energy Survey. Uses dark energy camera (DEcam). 5000 deg of survey over multiple bands (galaxies, clusters, distribution) - but this is another topic. They also had 30 square degrees of supernova - another topic. In 2015 started new program - gravitational waves stuff: GW program (her topic). Ended in 2020.
image
  • Galaxy-galaxy cluster, galaxy-galaxy lensing, cosmic shear. DES Y1 vs. Planck vs. DES Y1 + Planck - bigger discrepancy (?) as measurements get more precise.
image
image
  • How do grav waves help? “Standard sirens”. Use particle formula from Einstein GR for merger. Key feature: rate of change in frequency as orbits decay depends on combined masses and dependency is same with inverse power. So if detector can observe and you measure delta_freq then you can get the amplitude of the waves, compare with observed amplitude and that gives you distance and luminosity. Didn’t need calibration of the source (that introduces systematic uncertainties - so not needing it means more precise).
  • That image above (GW170817) was independently discovered around the same time.

Article 1: https://www.brandeis.edu/now/2017/october/gravitational-SoaresSantos-waves.html

  • “In the short term, we will gain new insights into neutron stars, which occur when giant stars 10 to 30 times as big as the sun collapse into objects about the size of the greater Boston metropolitan area. But over a longer period, gravitational waves may explain the universe’s continued expansion and the composition of dark energy, an elusive, mysterious substance that makes up roughly 70 percent of the universe.”
  • Ligo reported their BH merger, this new stuff is the neutron star merger. “The signals for black hole gravitational waves usually last less than a second while those from neutron stars persist for as long as a minute, providing reams more data.”
  • “But this time around, in addition to capturing these vibrations, [(vibrations from gravitational waves distorting spacetime)] Soares-Santos and her colleagues in Chile captured the waves’ optical signal. An image yields far more precise information than the sound recordings”

Article 2: https://lsa.umich.edu/physics/people/faculty/marcelle-soares-santos.html

  • The DEcam “has been successful in detecting the first neutron star collision ever observed, a discovery heralded as the Science breakthrough of the year, in 2017.”
  • “Soares-Santos research has recently been featured in the PBS documentary TV series Nova Wonders and has been covered by major news outlets worldwide.”

Slides Plan (~6 slides?)

  1. Gravitational waves intro and how they tall tell us about (1) composition of dark energy and (2) explain the universe’s expansion.
  2. Universe expansion: cosmological parameters
    1. Universe is expanding, relate this to cosmo parameters including Hubble
    2. Hubble parameter discrepancy (CMB, supernovae, …)
  3. Multi-messenger astronomy (c quoi)
  4. Dark Energy Survey
    1. DEcam - what is it, what is it looking for (there’s a few projects I think)
    2. Detected first neutron star collision: GW170817 (but independent discoveries?)