Distinguishing modified gravity models

Modified gravity models with screening in local environments appear in three different guises: chameleon, K-mouflage and Vainshtein mechanisms. We propose to look for differences between these classes of models by considering cosmological observations at low redshift. In particular, we analyse the redshift dependence of the fine structure constant and the proton to electron mass ratio in each of these scenarios. When the absorption lines belong to unscreened regions of space such as dwarf galaxies, a time variation would be present for chameleons. For both K-mouflage and Vainshtein mechanisms, the cosmological time variation of the scalar field is not suppressed in both unscreened and screened environments, therefore enhancing the variation of constants and their detection prospect. We also consider the time variation of the redshift of distant objects using their spectrocopic velocities. We find that models of the K-mouflage and Vainshtein types have very different spectroscopic velocities as a function of redshift and that their differences with the $\Lambda$-CDM template should be within reach of the future ELT- HIRES observations.Comment: 18 pages, 10 figure

Inflation from Tachyon Condensation, Large N Effects

Using only general properties of the tachyon potential we show that inflation may be generic when many branes and anti-branes become coincident. Inflation may occur because of: (1) the assistance of the many diagonal tachyon fields; (2) when the tachyons condense in a staggered fashion; or (3) when some of them condense very late. We point out that such inflation is in some sense a stringy implementation of chaotic inflation and may have important applications for ``regularizing'' a lopsided or singular cosmological compact surface.Comment: 24 pages, 1 figur

Non-canonical inflation coupled to matter

We compute corrections to the inflationary potential due to conformally coupled non-relativistic matter. We find that under certain conditions of the matter coupling, inflation may be interrupted abruptly. We display this in the superconformal Starobinsky model, where matter is conformally coupled to the Einstein frame metric. These corrections may easily stop inflation provided that there is an initial density of non-relativistic matter. Since these additional heavy degrees of freedom generically occur in higher dimension theories, for example as Kaluza-Klein modes, this effect can arise in multiple scenarios.Comment: Matches published versio

Supersymmetron

We consider a supersymmetric model of dark energy coupled to cold dark matter: the supersymmetron. In the absence of cold dark matter, the supersymmetron converges to a supersymmetric minimum with a vanishing cosmological constant. When cold dark matter is present, the supersymmetron evolves to a matter dependent minimum where its energy density does not vanish. In the early universe until the recent past of the Universe, the energy density of the supersymmetron is negligible compared to the cold dark matter energy density. Away from the supersymmetric minimum, the equation of state of the supersymmetron is constant and negative. When the supersymmetron reaches the neighbourhood of the supersymmetric minimum, its equation of state vanishes rapidly. This leads to an acceleration of the universe which is transient unless supersymmetry breaking induces a pure cosmological constant and acceleration of the Universe does not end. Moreover, we find that the mass of supersymmetron is always greater than the gravitino mass. As a result, the supersymmetron generates a short ranged fifth force which evades gravitational tests. On the other hand, we find that the supersymmetron may lead to relevant effects on large scale structures.Comment: published version, 11 page

Casimir, Gravitational and Neutron Tests of Dark Energy

We investigate laboratory tests of dark energy theories which modify gravity in a way generalising the inverse power law chameleon models. We make use of the tomographic description of such theories which captures $f(R)$ models in the large curvature limit, the dilaton and the symmetron. We consider their effects in various experiments where the presence of a new scalar interaction may be uncovered. More precisely, we focus on the Casimir, Eot-wash and neutron experiments. We show that dilatons, symmetrons and generalised chameleon models are efficiently testable in the laboratory. For generalised chameleons, we revise their status in the light of forthcoming Casimir experiments like CANNEX in Amsterdam and show that they are within reach of detection.Comment: 30 pages, 5 figure

One-electron atoms in screened modified gravity

In a large class of scalar-tensor theories that are potential candidates for dark energy, a nonminimal coupling between the scalar and the photon is possible. The presence of such an interaction grants us the exciting prospect of directly observing dark sector phenomenology in the electromagnetic spectrum. This paper investigates the behavior of one-electron atoms in this class of modified gravity models, exploring their viability as probes of deviations from general relativity in both laboratory and astrophysical settings. Building heavily on earlier studies, our main contribution is threefold: A thorough analysis finds additional fine-structure corrections previously unaccounted for, which now predict a contribution to the Lamb shift that is larger by nearly 4 orders of magnitude. In addition, they also predict a scalar-mediated photon-photon interaction, which now constrains the scalar's coupling to the photon independently of the matter coupling. This was not previously possible with atomic precision tests. Our updated constraints are $\log_{10}\beta_m \lesssim 13.4$ and $\log_{10}\beta_\gamma \lesssim 19.0$ for the matter and photon coupling, respectively, although these remain uncompetitive with bounds from other experiments. Second, we include the effects of the nuclear magnetic moment, allowing for the study of hyperfine structure and the 21 cm line, which hitherto have been unexplored in this context. Finally, we also examine how a background scalar leads to equivalence principle violations.Comment: Minor updates to match PRD versio

Electroweak Baryogenesis with Electroweak Strings

If stable electroweak strings are copiously produced during the electroweak phase transition, they may contribute significantly to the presently observed baryon to entropy ratio of the Universe. This analysis establishes the feasibility of implementing an electroweak baryogenesis scenario without a first order phase transition.Comment: BROWN-HET-862, 12 pages; use phyzz

Electroweak Baryogenesis with a Second Order Phase Transition

If stable electroweak strings are copiously produced during the electroweak phase transition, they may contribute significantly to the presently observed baryon to entropy ratio of the universe. This analysis establishes the feasibility of implementing an electroweak baryogenesis scenario without a first order phase transition.Comment: 7 pages, use phyzzx, BROWN-HET-86

Cosmic String Current Stability

The stability of fermionic charge carriers on cosmic strings is considered. We show that neutral fermion currents in cosmic strings are always chiral or time-like, in contrast to the case of bosonic currents. The spectrum of bound states on an abelian SO(10) string is determined both before and after the electroweak phase transition. We determine the mass acquired by the zero mode at this transition. A range of charge carrier scattering processes are considered and corresponding decay rates calculated. Couplings between the carriers and the electroweak sector generate scattering from the plasma which can thermalise some currents. If the zero mode is isolated from the electroweak sector, then it survives. Current-current scattering is considered, but found to be unimportant in realistic settings where the string density is low.Comment: RevTex, 28 pages, 3 figure

Pulsar Constraints on Screened Modified Gravity

We calculate the rate of energy loss from compact astrophysical objects due to a scalar field in screened modified gravity models of the chameleon, dilaton and symmetron types. The cosmological evolution of the field results in a time-variation of the scalar charge of screened objects implying the emission of scalar radiation. Focusing on binary objects, this leads to an additional decay in the orbital period complementing that due to the emission of gravitational waves. Using the Hulse-Taylor binary pulsar, the double pulsar PSR J0737-3039 and the pulsar-white dwarf system PSR J1738+033, we find a new observational bound on the time variation of the scalar charge of the earth in the Milky Way. We then translate this into a new bound on the range of the scalar interaction in the Milky Way. Ultimately, we find that pulsar tests are not competitive with current observational constraints.Comment: 23 pages, published versio