I am currently a postdoctoral researcher in theoretical physics, astrophysics, and cosmology at the Center for Particle Cosmology, University of Pennsylvania, Philadelphia, USA.

Previously, I was a postdoctoral researcher at the Institute of Cosmology and Gravitation, Portsmouth University, UK. Before that, I was a visiting fellow at the Perimeter Institute for Theoretical Physics, Waterloo, Ontario, Canada and a Ph.D. candidate at the Department of Applied Mathematics and Theoretical Physics, University of Cambridge, UK.

I am a theoretical physicist, and spend most of my time studying alternate theories of gravity that could potentially explain why the expansion of the universe is accelerating. I am particularly interested in theories with screening mechanisms such as symmetron and galileon gravity. A lot of my research focuses on looking for new and novel ways of testing these theories using stars and galaxies.

My research focuses on alternate theories of gravity to Einstein's general relativity. In particular, I am very interested in scalar-tensor theories of gravity with screening mechanisms such as the chameleon and symmetron models.

We were all taught in school that the universe is expanding but in the last decade or so there have been several observations that the expansion is actually accelerating. The universe is expanding faster and faster. "So what?" you might say, if it's expanding then what does it matter whether it’s accelerating or not? We all know that gravity is an attractive force that it pulls things together and it is the force that dominates the behaviour of the universe. Even if the universe is expanding we would expect gravity to pull everything together, not push it apart and so whatever is causing this expansion, it is very unlikely to be gravity. This mysterious mechanism driving the acceleration of the universe has come to be known as dark energy.

So what could possibly be causing this strange behaviour? One possibility is that there is some strange new type of matter that repels itself under gravity, but another possibility is that gravity itself behaves differently on cosmological scales to how it works in the solar system. It is this possibility that I am interested in and one goal when constructing new modified gravity models is to investigate whether or not they can drive the cosmic acceleration.

It's all well and good finding a modified theory of gravity that predicts the acceleration, but these theories are really modifications to gravity as described by General Relativity and Einstein's equations. Why would you want to do that, Einstein's theory of general relativity works so well? You might ask. Well it does, in fact it works so well that modifying it ad hoc usually violates the most stringent experiments testing its predictions in the solar system. Either that or the constraints are so tight that the new theories are unobservable. Einstein's equations explain everything from the perihelion of Mercury to the structure and evolution of our sun and so whatever we do to General Relativity on large scales we better have some way of recovering what we already know about our own solar system.

One thing you could do is make the modifications sub-dominant but this is counter-productive: It renders them negligible on all scales and so the resultant theory will have nothing to say about dark energy that general relativity doesn’t. What we need is some way to have large modifications that are somehow hidden in the solar system.

This is accomplished using screening mechanisms. These use non-linear effects to decouple solar system scale from cosmological ones so that the theory predicts no deviations from general relativity in the solar system but drastic differences when applied to the entire universe.

The problem then is how to test these theories if they behave identically to general relativity. Much of my time is spent trying to find new and novel ways of testing these theories.

The chameleon mechanism hides modifications of general relativity by adding a new particle---the chameleon---that blends in with its environment. Heavy objects like our Sun generate a gravitational field but a much weaker chameleon field. This is because the source for the field is not the entire object but only a very thin shell near the surface.

Different objects have different shell thicknesses; thin shells are screened and thick shells are not. Some of my previous research has been focused on deriving the altered properties of unscreened stars, which can exist in distant dwarf galaxies. Unscreened stars are brighter and hotter than general relativity predicts. This is because they have to burn nuclear fuel in their core at a higher rate to avoid collapsing and this causes energy to be released at a higher rate. Another interesting phenomena is that some stars pulsate faster. My collaborators and I were able to use this effect to rule out a large number of chameleon theories.

The Vainshtein mechanism hides the effects of modifications by introducing a new radius, the Vainshtein radius. Inside this radius, the theory looks like general relativity and the modifications are only felt outside. This mechanism is very efficient: the Vainshtein radius of the Sun encompasses the entire solar system.

Nevertheless, there are some theories we can test this using stars. In these theories, the Vainshtein mechanism makes gravity weaker inside stars and so they are dimmer and cooler than general relativity predicts. In addition to this, the weakening of gravity makes stars and gas clouds inside galaxies orbit the centre at a slower rate.

Disformal gravity is a strange theory that alters the structure of space-time so that the speed of gravity is different from the speed of light. I have worked on constraining this theory using solar system experiments and have elucidated the ways in which this theory changes the origin and evolution of the universe compared with general relativity and other dark energy models.

- Hector O. Silva, Jeremy Sakstein, Leonardo Gualtieri, Thomas P. Sotiriou & Emanuele Berti,
*Spontaneous scalarization of black holes and compact stars from a Gauss-Bonnet coupling*, arXiv: 1711.02080. - Jeremy Sakstein & Bhuvnesh Jain,
*Implications of the Neutron Star Merger GW170817 for Cosmological Scalar-Tensor Theories*, Phys.Rev.Lett. 119 (2017) no.25, 251303, arXiv: 1710.05893. - Jeremy Sakstein,
*Tests of Gravity with Future Space-Based Experiments*, arXiv: 1710.03156. - Clare Burrage & Jeremy Sakstein,
*Tests of Chameleon Gravity*, arXiv: 1709.09071. - Ruifeng Dong, Jeremy Sakstein & Dejan Stojkovic,
*Quasi-normal modes of black holes in scalar-tensor theories with non-minimal derivative couplings*, arXiv: 1704.02425. - Jeremy Sakstein & Adam R. Solomon,
*Baryogenesis in Lorentz-violating gravity theories*, Phys.Lett. B773 (2017) 186-190, arXiv: 1705.10695. - Jeremy Sakstein, Bhuvnesh Jain, Jeremy Heyl, & Lam Hui,
*Tests of Gravity Theories Using Supermassive Black Holes*, Astrophys.J. 844 (2017) no.1, L14, arXiv: 1704.02425. - Jeremy Sakstein & Mark Trodden,
*Baryogenesis via Dark Matter-Induced Symmetry Breaking in the Early Universe*, Phys.Lett. B774 (2017) 183-188, arXiv: 1703.10103. - Jeremy Sakstein, Eugeny Babichev, Kazuya Koyama, David Langlois & Ryo Saito,
*Towards Strong Field Tests of Beyond Horndeski Gravity Theories*, Phys.Rev. D95 (2017) no.6, 064013, arXiv: 1612.04263. - Jeremy Sakstein, Michael Kenna-Allison & Kazuya Koyama,
*Stellar Pulsations in Beyond Horndeski Gravity Theories*, JCAP 1703 (2017) no.03, 007, arXiv: 1611.01062. - Clare Burrage & Jeremy Sakstein,
*A Compendium of Chameleon Constraints*, JCAP 1611 (2016) no.11, 045, arXiv: 1609.01192. - Eugeny Babichev, Kazuya Koyama, David Langlois, Ryo Saito & Jeremy Sakstein,
*Relativistic Stars in Beyond Horndeski Theories*, Class.Quant.Grav. 33 (2016) no.23, 235014, arXiv: 1606.06627. - Jeremy Sakstein, Harry Wilcox, David Bacon, Kazuya Koyama & Robert C. Nichol,
*Testing Gravity Using Galaxy Clusters: New Constraints on Beyond Horndeski Theories*, JCAP 1607 (2016) no.07, 019, arXiv: 1603.06368. - Jeremy Sakstein & Kazuya Koyama,
*Testing the Vainshtein Mechanism using Stars and Galaxies*, Int.J.Mod.Phys. D24 (2015) 12, 1544021. - Jeremy Sakstein,
*Testing Gravity Using Dwarf Stars*, Phys.Rev. D92 (2015) 12, 124045, arXiv: 1511.01685. - Jeremy Sakstein,
*Hydrogen Burning in Low Mass Stars Constrains Alternative Gravity Theories*, Phys.Rev.Lett. 115 (2015) 201101, 051, arXiv: 1510.05964. - Jeremy Sakstein & Sarunas Verner,
*Disformal Gravity Theories: A Jordan Frame Analysis*, Phys.Rev. D92 (2015) 12, 123005, arXiv: 1509.05679. - Hiu Yan Ip, Jeremy Sakstein & Fabian Schmidt,
*Solar System Constraints on Disformal Gravity Theories*, JCAP 1510 (2015) 10, 051, arXiv: 1507.00568. - Kazuya Koyama & Jeremy Sakstein,
*Astrophysical Probes of the Vainshtein Mechanism: Stars and Galaxies*, Phys.Rev. D91 (2015) 12, 124066, arXiv: 1502.06872. - Jeremy Sakstein,
*Astrophysical Tests of Modified Gravity*, Ph. D. thesis, arXiv: 1502.04503. - Jeremy Sakstein,
*Towards Viable Cosmological Models of Disformal Theories of Gravity*, Phys.Rev. D91 (2015), arXiv: 1409.7296. - Jeremy Sakstein,
*Disformal Theories of Gravity: From the Solar System to Cosmology*, JCAP 1412 (2014), arXiv: 1409.1734. - Jeremy Sakstein, Bhuvnesh Jain, & Vinu Vikram,
*Detecting modified gravity in the stars*, Int.J.Mod.Phys. D23 (2014), arXiv: 1409.3708. - Vinu Vikram, Jeremy Sakstein, Charles Davis, & Andrew Neil,
*Astrophysical Tests of Modified Gravity: Stellar and Gaseous Rotation Curves in Dwarf Galaxies*, arXiv: 1407.6044. - Jeremy Sakstein,
*Stellar Oscillations in Modified Gravity*, Phys.Rev. D88 (2013), arXiv: 1309.0495. - Philippe Brax, Anne-Christine Davis, & Jeremy Sakstein,
*Dynamics of Supersymmetric Chameleons*, JCAP 1310 (2013), arXiv: 1302.3080. - Philippe Brax, Anne-Christine Davis, & Jeremy Sakstein,
*Pulsar Constraints on Screened Modified Gravity*, Class.Quant.Grav. 31 (2014), arXiv: 1301.5587. - Philippe Brax, Anne-Christine Davis, & Jeremy Sakstein,
*SUPER-Screening*, Phys.Lett. B719 (2013), arXiv: 1212.4392. - Bhuvnesh Jain, Vinu Vikram, & Jeremy Sakstein,
*Astrophysical Tests of Modified Gravity: Constraints from Distance Indicators in the Nearby Universe*, Astrophys.J. 779 (2013), arXiv: 1204.6044. - Anne-Christine Davis, Eugene A. Lim, Jeremy Sakstein, & Douglas Shaw,
*Modified Gravity Makes Galaxies Brighter*, Phys.Rev. D85 (2012), arXiv: 1102.5278. - Jeremy Sakstein, Antonio Pipino, Julien E.G. Devriendt, & Roberto Maiolino,
*The Origin and Evolution of the Mass-Metallicity Relation using GalICS*, Mon.Not.Roy.Astron.Soc. 410 (2011), arXiv:1008.4158.

A six hour lecture course aimed at postdocs and graduate students at the Institue of Cosmology and Gravitation, University of Portsmouth, UK. The course is an introduction to non-relativistic gravity with an aim towards testing alternative theories of gravity. The topics covered are: Newtonian and post-Newtonian general relativity, screening mechanisms, and non-relativistic stars.

The notes are available here.

**Testing Gravity Using Astrophysics:**

http://pirsa.org/16030076

A talk given at The Perimeter Institute for Theoretical Physics on 03/08/2016. The talk reviews my recent work testing beyond Horndeski theories using novel astrophysical probes such as dwarf stars.

**Aspects of Modified Gravity:**

http://pirsa.org/13100132

A talk given at The Perimeter Institute for Theoretical Physics on 10/31/2013. The talk is split into two halves: the first discusses my work on finding supersymmetric models of chameleon gravity and the second reviews my work on different astrophysical tests of generic theories using stars.

**Detecting Modified Gravity in the Stars:**

http://pirsa.org/13050009/

http://hosting.epresence.tv/CITA/1/watch/522.aspx

A talk given at The Perimeter Institute for Theoretical Physics on 05/14/2013 and the Canadian Center for Theoretical Astrophysics on 05/16/2013. The talk describes my work in deriving the properties of stars in chameleon gravity and how pulsating objects can be used to place new constraints on the theory. The constraints presented here are the strongest to-date.

**Tests of Cosmological Gravity**

View slides

An invited talk given at KIPAC Stanford, CA on 01/22/2018 and UC Irvine, CA on 01/23/2017. The talk discusses my work on testing alternative gravity theories by looking for offset supermassive black holes in galaxies falling into clusters. It then discusses the implications that the observation of gravitational waves and an optical counterpart from the neutron star merger GW170817 has for cosmological gravity theories.

**Screening Constraints and Cosmological Implications**

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An invited talk given at the workshop *Dark Energy in the Laboratory* held at the Lorentz center, Netherlands from 13-17 November 2017. The talk discusses the parameter space of chameleon models in the context of cosmology and current constraints on all scales. The current state of the field is summarized and future prospects are discussed.

**Synergizing Screening Mechanisms
on Different Scales**

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An invited talk given at the workshop *Probing the Dark Sector and General Relativity at all Scales* held at CERN, Geneva from 14-25 August 2017. The talk reviews several different astrophysical tests of chameleon and galileon gravity theories, and relates the constraints to laboratory, cosmological, and strong field tests.

**Testing Galileon Gravity Using Supermassive Black Holes**

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A talk given at the conference *PASCOS 2017* at Instituto de Fisica Teorica, Madrid on 20/06/2017. The talk discusses a new method for testing galileon gravity theories by looking for offset supermassive black holes in galaxies falling into clusters, and presents new constraints on the model parameters.

**Some Mechanisms for Baryogenesis**

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An invited talk given at the Institute of Cosmology and Gravitation, University of Portsmouth on 04/14/2017. The talk describes two new mechanisms that could produce the asymmetry between matter and antimatter in the early universe.

**Novel Tests of Gravity Using Astrophysics**

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An invited colloquium talk given at the University of Mississippi on 11/01/2016. The talk reviews dark energy and modified gravity before presenting my work on testing beyond Horndeski gravity using dwarf stars and neutron stars. The talk is aimed a broad audience from non-cosmological areas of physics.

**Unifying Small Scale Probes with Cosmology:
Astrophysical Tests of Gravity**

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An invited talk given at the workshop *Unifying Tests of General Relativity* held at Caltech from 19-21 July 2016. The talk reviews my work on testing gravity using dwarf stars and neutron stars and how the constraints can be connected to cosmology.

**Testing Gravity Using Astrophysics**

View slides

A talk given at the Department of Applied Mathematics and Theoretical Physics, University of Cambridge, UK on 05/09/2016. The talk reviews my work testing beyond Horndeski gravity theories using stars and galaxies.

**Testing Gravity Using Astrophysics**

View slides

A talk given at the ICG-KASI Collaboration Workshop on 09/17/2015. The talk reviews my work testing chameleon and Vainshtein screened theories using stars and galaxies.

**Astrophysical Tests of Gravity**

View slides

A colloquium talk given at the physics department at the University of Sheffield on 03/04/2015. The talk reviews my work over the last few years looking for probing chameleon and galileon gravity in using stars and galaxies.

**Disformal Theories of Gravity**

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Talk given at the University of Helsinki on 12/03/2014. The talk describes my work on disformal theories of gravity. The first half discusses the Newtonian behaviour in the solar system and shows how the theory can be screened in the solar system depending on the cosmology. The second half classifies the possible cosmological solutions using dynamical systems techniques.

**Hide and Seek: Screening Mechanisms Present and Future**

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An invited talk given at the workshop *Non-linear Structure in the Modified Universe* held in at the Lorentz Centre in Leiden from 14-18 July 2014. The talk reviews the current status of various screening mechanisms, identifies future directions in their study and discusses the future prospects for probing them on small scales.

**Scalar-Tensor Screening Mechanisms**

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An invited talk given at the workshop *Novel Probes of Dark Energy and Modified Gravity* at the University of Pennsylvania from 26-27 April 2014. The talk gives a general introduction to chameleon-like theories giving an overview of the current constraints and recent developments in the field.

xAct is an add-on package for Mathematica that performs tensor algebra. I have written a short tutorial that is aimed at researchers working in modified gravity and cosmology. The topics covered are: tensor manipulations (xTensor), equations of motion (xPert), coordinate systems (xCoba), cosmological perturbation theory (xPert), and relativistic stars.

xAct can be downloaded here.

The extra package xPand is also required and can be downloaded here.

My tutorial is available here.

Download my CV | |
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Email: |
sakstein [at] physics.upenn.edu |

Mailing: |
Center for Particle Cosmology Department of Physics & Astronomy 209 South 33rd Street University of Pennsylvania Philadelphia, PA 19104-6396 USA |