arXiv subjects
Publications and source records attributed to Martin S. Sloth.
Lecture notes on inflation. The lectures are three double lectures, held for the first time at the Nordita Winter School 2024 - Particle Physics and Cosmology, covering an advanced introduction to the theory of primordial inflation, as well as the linear and non-linear perturbation theory of slow-roll inflation/quasi-de Sitter spacetimes.
We point out that constraints on $\Delta N_\mathrm{eff}$ reported by the ACT collaboration in their DR6 data release are surprisingly sensitive to the assumptions made about the initial power spectrum from inflation. The ACT collaboration reports no evidence of new light degrees of freedom alongside a low value of the expansion rate, thus confirming the Hubble tension. However, as we show here, when considering self-interacting dark radiation and including running, $\alpha_s$, and running of the running, $\beta_s$, of the spectral index $n_s$, the picture changes significantly. Confronting this extended model with Planck, ACT DR6, DESI DR2, and uncalibrated Pantheon+ data, we find the significantly relaxed bound $\Delta N_\text{eff}< 0.58$ at 95$\%$ CL, together with a $2.9 \sigma$ ($2.6 \sigma$) preference for $\alpha_s>0$ ($\beta_s>0$), while the Hubble tension is reduced to $2.2 \sigma$ with only three more parameters compared to $\Lambda$CDM. If the dark radiation fluid is initially coupled to dark matter, and undergoes dark radiation-matter decoupling (DRMD) around matter-radiation equality, predicting dark acoustic oscillations with drag horizon $r_{d,\mathrm{DAO}} \approx 60 \,\mathrm{Mpc}/h$, the bound is further relaxed to $\Delta N_\text{eff}< 0.68$ at 95$\%$ CL, reducing the Hubble tension below $2\sigma$. We also discuss how $\alpha_s$ and $\beta_s$ could naturally appear in inflationary scenarios, possibly connected to the end of a first act of inflation. In this case dark radiation is mostly probed by scales covered by Planck and DESI, while smaller scales carry information on inflationary dynamics.
The Hubble tension and the recently reported anomaly in data from the Dark Energy Spectroscopic Instrument (DESI) are considered to pose serious challenges to the standard $\Lambda$CDM model of cosmology. In this work, we show that resolving the Hubble tension with a scenario featuring dark radiation-matter decoupling (DRMD) predicts the presence of dark acoustic oscillations (DAO) similar in scale to baryon acoustic oscillations (BAO). Using an inference independent of large-scale structure data, relying only on Planck measurements of the cosmic microwave background and SH$0$ES-calibrated supernova data, we find evidence for a DAO signal with drag-horizon scale $r_{d,\mathrm{DAO}} \in[54,65]\,\mathrm{Mpc}/h$ ($68\%\,\mathrm{C.I.}$) and amplitude $A_\mathrm{DAO} \in [0.02,0.05]$ ($68\%\,\mathrm{C.I.}$). These predictions provide a concrete target for current and upcoming large-scale structure surveys, including DESI, Euclid, and the Roman Space Telescope. Remarkably, the predicted DAO properties are consistent with those required to explain the DESI anomaly, offering both an alternative to evolving dark energy and a preliminary validation of the relevance of a dark radiation-matter decoupling scenario for addressing the Hubble tension.
DESI DR2 data have been widely interpreted as evidence for late-time evolving dark energy (DE) with an apparent phantom crossing. Here we investigate an alternative explanation, based on early-Universe physics. If dark acoustic oscillations (DAO) are close in scale to baryon acoustic oscillations (BAO), they can bias the extraction of the BAO scale from the peak in the galaxy correlation function. This leads to an apparent shift in the inferred distance if the superposition of BAO and DAO features is misinterpreted as being due to BAO only. Taking this shift into account, we find that a DAO with percent-level amplitude can reconcile DESI DR2 with Planck 2018 as well as Pantheon+ supernovae data, with fit improvement at a similar level compared to evolving DE. Notably, a DAO feature with the required properties has been predicted in a previously proposed scenario that resolves the Hubble tension via a pre-recombination decoupling of dark matter and dark radiation (DRMD). The presence of a DAO feature close to the BAO peak can be scrutinized with future full-shape galaxy clustering data from DESI and Euclid.
We present a microscopic model of the dark sector that resolves the Hubble tension within standard current data sets (Planck 2018, Pantheon+ and DESI DR2 BAO) based on well-known fundamental principles, gauge symmetry and spontaneous symmetry breaking. It builds on the Hot New Early Dark Energy (Hot NEDE) setup, featuring a dark $SU(N)$ gauge symmetry broken to $SU(N-1)$ in a supercooled phase transition that creates a thermal bath of self-interacting dark radiation in the epoch between Big Bang Nucleosynthesis and recombination. Adding a fermion multiplet charged under the gauge symmetry provides a naturally stable component of dark matter that interacts with dark radiation. Spontaneous symmetry breaking predicts a decoupling of this interaction once the dark sector cools down, that we refer to as dark radiation matter decoupling (DRMD). We also provide a simplified DRMD model that captures the essential features of the full theory while retaining additional falsifiable predictions. Using the data sets stated above, we find agreement with the SH${}_0$ES determination of $H_0$ at the 1.4$\sigma$ level, compared to a 5.7$\sigma$ tension in $\Lambda$CDM, thereby providing a resolution of the Hubble tension.
I demonstrate that soft graviton modes in de Sitter spacetimes are the Goldstone modes of the spontaneously broken asymptotic symmetry group of de Sitter space. I then show that any local measurement, including the effects of the environment, will collapse the symmetric state onto the broken state in the large volume limit. In any discussion involving observers, de Sitter spacetimes are, therefore, best described globally by the broken phase, while local observers, in the small volume limit, can not discriminate between different degenerate global vacuum states and are therefore best described by the symmetric state. As a consequence, a small Hubble-sized local region initially in the symmetric state will, after a time scale corresponding to the Page time of de Sitter space, have expanded to a large region in the broken state. This illuminates the physical nature of soft graviton modes in de Sitter spacetimes.
We explore the possibility that exotic forms of dark matter could expose humans on Earth or on prolonged space travel to a significant radiation dose. The radiation exposure from dark matter interacting with nuclei in the human body is generally assumed to be negligible compared to other sources of background radiation. However, as we discuss here, current data allow for dark matter models where this is not necessarily true. In particular, if dark matter is heavier and more strongly interacting than weakly interacting massive particle dark matter, it could act as ionizing radiation and deposit a significant amount of radiation energy in all or part of the human population, similar to or even exceeding the known radiation exposure from other background sources. Conversely, the non-observation of such an exposure can be used to constrain this type of heavier and more strongly interacting dark matter. We first consider the case where dark matter scatters elastically and identify the relevant parameter space in a model-independent way. We also discuss how previous bounds from cosmological probes, as well as atmospheric and space-based detectors, might be avoided, and how a re-analysis of existing radiation data, along with a simple experiment monitoring ionizing radiation in space with a lower detection threshold, could help constrain part of this parameter space. We finally propose a hypothetical dark matter candidate that scatters inelastically and argue that, in principle, one per mille of the Earth's population could attain a significant radiation dose from such a dark matter exposure in their lifetime.
Cold New Early Dark Energy (Cold NEDE) addresses the Hubble tension through a triggered vacuum phase transition in the dark sector. In this paper, we constrain a phenomenological fluid model using recent cosmic microwave background likelihoods based on Planck NPIPE data alongside baryonic acoustic oscillations (BAO) and supernovae data from Pantheon+. Exploiting the enhanced constraining power of the datasets, we introduce and study an extended version of the NEDE fluid model in which the equation of state parameter $w_\mathrm{NEDE}$, characterizing the post-phase transition fluid, is allowed to evolve with non-vanishing derivatives ${d}w_\mathrm{NEDE}/d\ln a$ and ${d^2}w_\mathrm{NEDE}/{d}(\ln a)^2$. Our results indicate that data is compatible with a rather simple time dependence that could arise from a mixture of radiation and a stiff fluid. With the updated datasets, the base and extended models still show a significant reduction of the DMAP tension from $6.3 \sigma$ in $\Lambda$CDM down to $3.5\sigma$ with a small simultaneous reduction of the $S_8$ tension, slightly improving over recent findings for the axion-like early dark energy model. Finally, we also provide a first test of the model against new BAO data from the Dark Energy Spectroscopic Instrument (DESI) survey. Replacing the previous BAO constraints in our analysis with the new ones, the tension is further reduced to $2.6 \sigma$, reaffirming the Cold NEDE model as a promising solution to the Hubble tension.
We propose a simple model that can alleviate the $H_0$ tension while remaining consistent with big bang nucleosynthesis (BBN). It is based on a dark sector described by a standard Lagrangian featuring a $SU(N)$ gauge symmetry with $N\geq3$ and a massive scalar field with a quartic coupling. The scalar acts as dark Higgs leading to spontaneous symmetry breaking $SU(N)\to SU(N\!-\!1)$ via a first-order phase transition \`a la Coleman-Weinberg. This set-up naturally realizes previously proposed scenarios featuring strongly interacting dark radiation (SIDR) with a mass threshold within hot new early dark energy (NEDE). For a wide range of reasonable model parameters, the phase transition occurs between the BBN and recombination epochs and releases a sufficient amount of latent heat such that the model easily respects bounds on extra radiation during BBN while featuring a sufficient SIDR density around recombination for increasing the value of $H_0$ inferred from the cosmic microwave background. Our model can be summarized as a natural mechanism providing two successive increases in the effective number of relativistic degrees of freedom after BBN but before recombination $\Delta N_\mathrm{BBN} \to \Delta N_\mathrm{NEDE} \to \Delta N_\mathrm{IR}$ alleviating the Hubble tension. The first step is related to the phase transition and the second to the dark Higgs becoming non-relativistic. This set-up predicts further signatures, including a stochastic gravitational wave background and features in the matter power spectrum that can be searched for with future pulsar timing and Lyman-$\alpha$ forest measurements.
It has recently been speculated that the NANOGrav observations point towards a first-order phase transition in the dark sector at the GeV scale [1]. Here, we show that such a phase transition might already have been predicted in the Hot New Early Dark Energy model (Hot NEDE) [2],[3]. There, it was argued that two dark sector phase transitions are the signature of neutrino mass generation through the inverse seesaw mechanism. In particular, an IR phase transition serves a double purpose by resolving the Hubble tension through an energy injection and generating the Majorana mass entry in the inverse seesaw mixing matrix. This usual NEDE phase transition is then accompanied by a UV counterpart, which generates the heavy Dirac mass entry in the inverse seesaw mass matrix of a right-handed neutrino. Here, we investigate if the UV phase transition of the Hot NEDE model can occur at the GeV scale in view of the recent NANOGrav observations.
New Early Dark Energy introduces a new phase of dark energy that decays in a fast-triggered phase transition around matter-radiation equality. The presence of a trigger mechanism sets it apart from other early dark energy models. Here, we will argue that New Early Dark Energy offers a simple and natural framework to extend $\Lambda$CDM while also providing a pathway to resolving the $H_0$ tension alongside its smaller cousin, the $S_8$ tension. At the microscopic level, we discuss the possibility that the trigger is either given by an ultralight scalar field or a dark sector temperature. In both cases, it prompts the transition of an $\mathrm{eV}$-mass scalar field from its false to its true minimum. Furthermore, we argue that the same phase transition could give rise to a dynamic process for generating neutrino masses.
In this work, we show that the Cold New Early Dark Energy (Cold NEDE) model in its original form can solve both the Hubble tension and the $S_8$ tension without adding any new ingredients at the fundamental level. So far, it was assumed that the trigger field in the Cold NEDE model is completely subdominant. However, relaxing this assumption and letting the trigger field contribute a mere $0.5\%$ of the total energy density leads to a resolution of the $S_8$ tension while simultaneously improving it as a solution to the $H_0$ tension. Fitting this model to baryonic acoustic oscillations, large-scale-structure, supernovae (including a SH0ES prior), and cosmic microwave background data, we report a preferred NEDE fraction of $f_\mathrm{NEDE}= 0.134^{+0.032}_{-0.025}$ ($68\%$ C.L.), lifting its Gaussian evidence for the first time above $5\sigma$ (up from $4 \sigma$ when the trigger contribution to dark matter is negligible). At the same time, we find the new concordance values $H_0 = 71.71 \pm 0.88 \,\mathrm{km}\, \mathrm{sec}^{-1}\, \mathrm{Mpc}^{-1}$ and $S_8 = 0.793 \pm 0.018$. Excluding large-scale structure data and the SH$_0$ES prior, both Gaussian tensions are reduced below the $2 \sigma$ level.
We examine further the ability of the New Early Dark Energy model (NEDE) to resolve the current tension between the Cosmic Microwave Background (CMB) and local measurements of $H_0$ and the consequences for inflation. We perform new Bayesian analyses, including the current datasets from the ground-based CMB telescopes Atacama Cosmology Telescope (ACT), the South Pole Telescope (SPT), and the BICEP/Keck telescopes, employing an updated likelihood for the local measurements coming from the S$H_0$ES collaboration. Using the S$H_0$ES prior on $H_0$, the combined analysis with Baryonic Acoustic Oscillations (BAO), Pantheon, Planck and ACT improves the best-fit by $Δχ^2 = -15.9$ with respect to $Λ$CDM, favors a non-zero fractional contribution of NEDE, $f_{\rm NEDE} > 0$, by $4.8σ$, and gives a best-fit value for the Hubble constant of $H_0 = 72.09$ km/s/Mpc (mean $71.48_{-0.81}^{+0.79}$ with $68\%$ C.L.). A similar analysis using SPT instead of ACT yields consistent results with a $Δχ^2 = - 23.1$ over $Λ$CDM, a preference for non-zero $f_{\rm NEDE}$ of $4.7σ$ and a best-fit value of $H_0=71.77$ km/s/Mpc (mean $71.43_{-0.84}^{+0.84}$ with $68\%$ C.L.). We also provide the constraints on the inflation parameters $r$ and $n_s$ coming from NEDE, including the BICEP/Keck 2018 data, and show that the allowed upper value on the tensor-scalar ratio is consistent with the $Λ$CDM bound, but, as also originally found, with a more blue scalar spectrum implying that the simplest curvaton model is now favored over the Starobinsky inflation model.
Recent interest in New Early Dark Energy (NEDE), a cosmological model with a vacuum energy component decaying in a triggered phase transition around recombination, has been sparked by its impact on the Hubble tension. Previous constraints on the model parameters were derived in a Bayesian framework with Markov-chain Monte Carlo (MCMC) methods. In this work, we instead perform a frequentist analysis using the profile likelihood in order to assess the impact of prior volume effects on the constraints. We constrain the maximal fraction of NEDE $f_\mathrm{NEDE}$, finding $f_\mathrm{NEDE}=0.076^{+0.040}_{-0.035}$ at $68 \%$ CL with our baseline dataset and similar constraints using either data from SPT-3G, ACT or full-shape large-scale structure, showing a preference over $Λ$CDM even in the absence of a SH0ES prior on $H_0$. While this is stronger evidence for NEDE than obtained with the corresponding Bayesian analysis, our constraints broadly match those obtained by fixing the NEDE trigger mass. Including the SH0ES prior on $H_0$, we obtain $f_\mathrm{NEDE}=0.136^{+0.024}_{-0.026}$ at $68 \%$ CL. Furthermore, we compare NEDE with the Early Dark Energy (EDE) model, finding similar constraints on the maximal energy density fractions and $H_0$ in the two models. At $68 \%$ CL in the NEDE model, we find $H_0 = 69.56^{+1.16}_{-1.29} \text{ km s}^{-1}\text{ Mpc}^{-1}$ with our baseline and $H_0 = 71.62^{+0.78}_{-0.76} \text{ km s}^{-1}\text{ Mpc}^{-1}$ when including the SH0ES measurement of $H_0$, thus corroborating previous conclusions that the NEDE model provides a considerable alleviation of the $H_0$ tension.
Hot new early dark energy describes a supercooled, first-order phase transition that takes place at sub-eV temperatures in the dark sector. It lowers the sound horizon, which provides a possible solution to the Hubble tension, and, at the same time, it can explain the neutrino masses through the inverse seesaw mechanism by making a set of sterile Majorana fermions massive. First, we argue that this scenario strengthens existing cosmological bounds on the heaviest neutrino mass. This, in turn, constrains the dark sector temperature, which provides us in total with two falsifiable predictions. In a second step, we discuss the phenomenological consequences of embedding hot new early dark energy in a larger gauge group that is partially broken above the TeV scale. This novel theory, which could even be motivated independently of the Hubble tension, completes the high-energy corner of the inverse seesaw mechanism and explains the mass of a dark matter candidate that can be produced through gravitational interactions at high energies.
We calculate the primordial correlation of gravitons with an abelian gauge field non-minimally coupled through a dynamical dilaton field or a volume moduli during inflation in the early universe. In particular, we compute the cross-correlation of a tensor mode with two gauge field modes and the corresponding correlation functions for the associated magnetic and electric fields using the in-in formalism. Moreover, using semi-classical methods, we show that the three-point cross-correlation functions satisfy new consistency relations (soft theorems) in the squeezed limit. Our findings exhibit a complete agreement of the full in-in results with the new consistency relations. An interesting consequence of our scenario is the possibility of a novel correlation of the primordial tensor mode with the primordial curvature perturbation induced by higher order quantum gravity corrections. The anisotropic background created by long wavelength gauge field modes makes this correlation function non-vanishing. Finally, we discuss how these three-point correlation functions are imprinted on cosmological observables today and the applications to scenarios of inflationary magnetogenesis.
New early dark energy (NEDE) makes the cosmic microwave background consistent with a higher value of the Hubble constant inferred from supernovae observations. It is an improvement over the old early dark energy model (EDE) because it explains naturally the decay of the extra energy component in terms of a vacuum first-order phase transition that is triggered by a subdominant scalar field at zero temperature. With hot NEDE, we introduce a new mechanism to trigger the phase transition. It relies on thermal corrections that subside as a subdominant radiation fluid in a dark gauge sector cools. We explore the phenomenology of hot NEDE and identify the strong supercooled regime as the scenario favored by phenomenology. In a second step, we propose different microscopic embeddings of hot NEDE. This includes the (non-)Abelian dark matter model, which has the potential to also resolve the LSS tension through interactions with the dark radiation fluid. We also address the coincidence problem generically present in EDE models by relating NEDE to the mass generation of neutrinos via the inverse seesaw mechanism. We finally propose a more complete dark sector model, which embeds the NEDE field in a larger symmetry group and discuss the possibility that the hot NEDE field is central for spontaneously breaking lepton number symmetry.
Recently a full-shape analysis of large-scale structure (LSS) data was employed to provide new constraints on a class of Early Dark Energy (EDE) models. In this note, we derive similar constraints on New Early Dark Energy (NEDE) using the publicly available PyBird code, which makes use of the effective field theory of LSS. We study the NEDE base model with the fraction of NEDE and the trigger field mass as two additional parameters allowed to vary freely while making simplifying assumptions about the decaying fluid sector. Including the full-shape analysis of LSS together with measurements of the cosmic microwave background (CMB), baryonic acoustic oscillations (BAO) and supernovae (SN) data, we report $ H_0= 71.2 \pm 1.0~\textrm{km}\, \textrm{s}^{-1}\, \textrm{Mpc}^{-1}$ ($68 \%$ C.L.) together with a $\simeq 4 \, σ$ evidence for a non-vanishing fraction of NEDE. This is an insignificant change to the value previously found without full-shape LSS data, $ H_0= 71.4 \pm 1.0~\textrm{km}\, \textrm{s}^{-1}\, \textrm{Mpc}^{-1} $ ($68 \%$ C.L.). As a result, while the NEDE fit cannot be improved upon the inclusion of additional LSS data, it is also not adversely affected by it, making it compatible with current constraints from LSS data. In fact, we find evidence that the effective field theory of LSS acts in favor of NEDE.