On the kinematic cosmic dipole tension

Our motion through the Universe generates a dipole in the temperature anisotropies of the Cosmic Microwave Background (CMB) and also in the angular distribution of sources. If the cosmological principle is valid, these two dipoles are directly linked, such that the amplitude of one determines that of the other. However, it is a longstanding problem that number counts of radio sources and of quasars at low and intermediate redshifts exhibit a dipole that is well aligned with that of the CMB but with about twice the expected amplitude, leading to a tension reaching up to $4.9 \sigma$. In this paper, we revisit the theoretical derivation of the dipole in the sources number counts, explicitly accounting for the redshift evolution of the population of sources. We argue that if the spectral index and magnification bias of the sources vary with redshift, the standard theoretical description of the dipole may be inaccurate. We provide an alternative expression which does not depend on the spectral index, but instead on the time evolution of the population of sources. We then determine the values that this evolution rate should have in order to remove the tension with the CMB dipole.Comment: 11 pages, 8 figures, typo corrected in Eq. (28), (43). Subsequent Eqs. (54), (56), (59), Fig. 7 and 8 adapted with respect to v

Statistical effects of the observer's peculiar velocity on source number counts

The velocity of the Sun with respect to the cosmic microwave background (CMB) can be extracted from the CMB dipole, provided its intrinsic dipole is assumed to be small in comparison. This interpretation is consistent, within fairly large error bars, with the measurement of the correlations between neighboring CMB multipoles induced by the velocity of the observer, which effectively breaks isotropy. In contrast, the source number count dipole was reported to privilege a velocity of the observer with an amplitude which is about twice as large as the one extracted from the entirely kinematic interpretation of the CMB dipole, with error bars which indicate a more and more significant tension. In this work, we study the effect of the peculiar velocity of the observer on correlations of nearby multipoles in the source number counts. We provide an unbiased estimator for the kinetic dipole amplitude, which is proportional to the peculiar velocity of the observer and we compute the expected signal to noise ratio. Near future experiments can achieve better than 5$\%$ constraints on the velocity of the Sun with our estimator.Comment: 12 pages, 1 figure, matches published versio

Scalar Čerenkov radiation from high-energy cosmic rays

As first noted by Robert Wagoner in the 1970s, if a scalar field is nonminimally coupled to the Ricci scalar and propagates at subluminal speeds, then there exists the possibility of scalar Cerenkov radiation from a moving particle. The mere observation of high-energy cosmic rays could in principle rule out the existence of such scalar fields since any particle moving faster than scalar perturbations would lose energy in the form of scalar waves until it moves slower than those. We compute in detail the energy loss to scalar waves and find that it scales with the square of the ultraviolet (UV) cutoff frequency of the effective field theory (EFT) of gravity. For dark-energy-motivated EFTs, the UV cutoff can be low, in which case that energy loss could always be negligible. In contrast, if viewed as a covariant theory valid at all scales or as an EFT valid at higher energies, perhaps even all the way up to the Planck scale, as may be the case if motivated by quantum-gravity perspectives, then the energy loss to scalar waves may diverge or become dramatically large. In this case, high-energy cosmic rays of extragalactic origin stringently constrain any conformally coupled scalar fields with noncanonical kinetic terms, although a minimum scalar phase velocity is required to trust the EF

Scalar and tensor gravitational waves

In dark-energy models where a scalar field is nonminimally coupled to the spacetime geometry, gravitational waves are expected to be supplemented with a scalar mode. Such scalar waves may interact with the standard tensor waves, thereby affecting their observed amplitude and polarization. Understanding the role of scalar waves is thus essential in order to design reliable gravitational-wave probes of dark energy and gravity beyond general relativity. In this article, we thoroughly investigate the propagation of scalar and tensor waves in the subset of Horndeski theories in which tensor waves propagate at the speed of light. We work at linear order in scalar and metric perturbations, in the eikonal regime, and for arbitrary scalar and spacetime backgrounds. We diagonalize the system of equations of motion and identify the physical tensor mode, which differs from the metric perturbation. We find that interactions between scalar and tensor waves generally depend on the scalar propagation speed. If the scalar waves are luminal or quasiluminal, then interactions are negligible. In the subluminal case, scalar-tensor interactions are effectively suppressed due to the incoherence of the wave’s phase

Scalar and tensor gravitational waves

In dark-energy models where a scalar field is nonminimally coupled to the spacetime geometry, gravitational waves are expected to be supplemented with a scalar mode. Such scalar waves may interact with the standard tensor waves, thereby affecting their observed amplitude and polarization. Understanding the role of scalar waves is thus essential in order to design reliable gravitational-wave probes of dark energy and gravity beyond general relativity. In this article, we thoroughly investigate the propagation of scalar and tensor waves in the subset of Horndeski theories in which tensor waves propagate at the speed of light. We work at linear order in scalar and metric perturbations, in the eikonal regime, and for arbitrary scalar and spacetime backgrounds. We diagonalize the system of equations of motion and identify the physical tensor mode, which differs from the metric perturbation. We find that interactions between scalar and tensor waves generally depend on the scalar propagation speed. If the scalar waves are luminal or quasiluminal, then interactions are negligible. In the subluminal case, scalar-tensor interactions are effectively suppressed due to the incoherence of the wave's phases.Comment: 12+6 pages, 1 figure. v2: extended results for subluminal scalar waves, matches published version in PR

Cosmology with the Laser Interferometer Space Antenna

254 pags:, 44 figs.The Laser Interferometer Space Antenna (LISA) has two scientific objectives of cosmological focus: to probe the expansion rate of the universe, and to understand stochastic gravitational-wave backgrounds and their implications for early universe and particle physics, from the MeV to the Planck scale. However, the range of potential cosmological applications of gravitational-wave observations extends well beyond these two objectives. This publication presents a summary of the state of the art in LISA cosmology, theory and methods, and identifies new opportunities to use gravitational-wave observations by LISA to probe the universe.This work is partly supported by: A.G. Leventis Foundation; Academy of Finland Grants 328958 and 345070; Alexander S. Onassis Foundation, Scholarship ID: FZO 059-1/2018-2019; Amaldi Research Center funded by the MIUR program “Dipartimento di Eccellenza” (CUP: B81I18001170001); ASI Grants No. 2016-24-H.0 and No. 2016-24-H.1-2018; Atracción de Talento Grant 2019-T1/TIC-15784; Atracción de Talento contract no. 2019-T1/TIC-13177 granted by the Comunidad de Madrid; Ayuda ‘Beatriz Galindo Senior’ by the Spanish ‘Ministerio de Universidades’, Grant BG20/00228; Basque Government Grant (IT-979-16); Belgian Francqui Foundation; Centre national d’Etudes spatiales; Ben Gurion University Kreitman Fellowship, and the Israel Academy of Sciences and Humanities (IASH) & Council for Higher Education (CHE) Excellence Fellowship Program for International Postdoctoral Researchers; Centro de Excelencia Severo Ochoa Program SEV-2016-0597; CERCA program of the Generalitat de Catalunya; Cluster of Excellence “Precision Physics, Fundamental Interactions, and Structure of Matter” (PRISMA? EXC 2118/1); Comunidad de Madrid, Contrato de Atracción de Talento 2017-T1/TIC-5520; Czech Science Foundation GAČR, Grant No. 21-16583M; Delta ITP consortium; Department of Energy under Grant No. DE-SC0008541, DE-SC0009919 and DESC0019195; Deutsche Forschungsgemeinschaft (DFG), Project ID 438947057; Deutsche Forschungsgemeinschaft under Germany’s Excellence Strategy - EXC 2121 Quantum Universe - 390833306; European Structural and Investment Funds and the Czech Ministry of Education, Youth and Sports (Project CoGraDS - CZ.02.1.01/0.0/0.0/15 003/0000437); European Union’s H2020 ERC Consolidator Grant “GRavity from Astrophysical to Microscopic Scales” (Grant No. GRAMS-815673); European Union’s H2020 ERC, Starting Grant Agreement No. DarkGRA-757480; European Union’s Horizon 2020 programme under the Marie Sklodowska-Curie Grant Agreement 860881 (ITN HIDDeN); European Union’s Horizon 2020 Research and Innovation Programme Grant No. 796961, “AxiBAU” (K.S.); European Union’s Horizon 2020 Research Council grant 724659 MassiveCosmo ERC-2016-COG; FCT through national funds (PTDC/FIS-PAR/31938/2017) and through project “BEYLA – BEYond LAmbda” with Ref. Number PTDC/FIS-AST/0054/2021; FEDER-Fundo Europeu de Desenvolvimento Regional through COMPETE2020 - Programa Operacional Competitividade e Internacionalização (POCI-01-0145- FEDER-031938) and research Grants UIDB/04434/2020 and UIDP/04434/2020; Fondation CFM pour la Recherche in France; Foundation for Education and European Culture in Greece; French ANR project MMUniverse (ANR-19-CE31-0020); FRIA Grant No.1.E.070.19F of the Belgian Fund for Research, F.R. S.-FNRS Fundação para a Ciência e a Tecnologia (FCT) through Contract No. DL 57/2016/CP1364/ CT0001; Fundação para a Ciência e a Tecnologia (FCT) through Grants UIDB/04434/2020, UIDP/04434/ 2020, PTDC/FIS-OUT/29048/2017, CERN/FIS-PAR/0037/2019 and “CosmoTests – Cosmological tests of gravity theories beyond General Relativity” CEECIND/00017/2018; Generalitat Valenciana Grant PROMETEO/2021/083; Grant No. 758792, project GEODESI; Government of Canada through the Department of Innovation, Science and Economic Development and Province of Ontario through the Ministry of Colleges and Universities; Grants-in-Aid for JSPS Overseas Research Fellow (No. 201960698); I?D Grant PID2020-118159GB-C41 of the Spanish Ministry of Science and Innovation; INFN iniziativa specifica TEONGRAV; Israel Science Foundation (Grant No. 2562/20); Japan Society for the Promotion of Science (JSPS) KAKENHI Grant Nos. 20H01899 and 20H05853; IFT Centro de Excelencia Severo Ochoa Grant SEV-2; Kavli Foundation and its founder Fred Kavli; Minerva Foundation; Ministerio de Ciencia e Innovacion Grant PID2020-113644GB-I00; NASA Grant 80NSSC19K0318; NASA Hubble Fellowship grants No. HST-HF2-51452.001-A awarded by the Space Telescope Science Institute with NASA contract NAS5-26555; Netherlands Organisation for Science and Research (NWO) Grant Number 680-91-119; new faculty seed start-up grant of the Indian Institute of Science, Bangalore, the Core Research Grant CRG/2018/002200 of the Science and Engineering; NSF Grants PHY-1820675, PHY-2006645 and PHY-2011997; Polish National Science Center Grant 2018/31/D/ ST2/02048; Polish National Agency for Academic Exchange within the Polish Returns Programme under Agreement PPN/PPO/2020/1/00013/U/00001; Pró-Reitoria de Pesquisa of Universidade Federal de Minas Gerais (UFMG) under Grant No. 28359; Ramón y Cajal Fellowship contract RYC-2017-23493; Research Project PGC2018-094773-B-C32 [MINECO-FEDER]; Research Project PGC2018-094773-B-C32 [MINECO-FEDER]; ROMFORSK Grant Project. No. 302640; Royal Society Grant URF/R1/180009 and ERC StG 949572: SHADE; Shota Rustaveli National Science Foundation (SRNSF) of Georgia (Grant FR/18-1462); Simons Foundation/SFARI 560536; SNSF Ambizione grant; SNSF professorship Grant (No. 170547); Spanish MINECO’s “Centro de Excelencia Severo Ochoa” Programme Grants SEV-2016- 0597 and PID2019-110058GB-C22; Spanish Ministry MCIU/AEI/FEDER Grant (PGC2018-094626-BC21); Spanish Ministry of Science and Innovation (PID2020-115845GB-I00/AEI/10.13039/ 501100011033); Spanish Proyectos de I?D via Grant PGC2018-096646-A-I00; STFC Consolidated Grant ST/T000732/1; STFC Consolidated Grants ST/P000762/1 and ST/T000791/1; STFC Grant ST/ S000550/1; STFC Grant ST/T000813/1; STFC Grants ST/P000762/1 and ST/T000791/1; STFC under the research Grant ST/P000258/1; Swiss National Science Foundation (SNSF), project The Non-Gaussian Universe and Cosmological Symmetries, Project Number: 200020-178787; Swiss National Science Foundation Professorship Grants No. 170547 and No. 191957; SwissMap National Center for Competence in Research; “The Dark Universe: A Synergic Multi-messenger Approach” Number 2017X7X85K under the MIUR program PRIN 2017; UK Space Agency; UKSA Flagship Project, Euclid.Peer reviewe