arXiv subjects

Tommi Tenkanen

Publications and source records attributed to Tommi Tenkanen.

At least 19 recordsLinked to original sources

arXiv2021

The First Three Seconds: a Review of Possible Expansion Histories of the Early Universe

It is commonly assumed that the energy density of the Universe was dominated by radiation between reheating after inflation and the onset of matter domination 54,000 years later. While the abundance of light elements indicates that the Universe was radiation dominated during Big Bang Nucleosynthesis (BBN), there is scant evidence that the Universe was radiation dominated prior to BBN. It is therefore possible that the cosmological history was more complicated, with deviations from the standard radiation domination during the earliest epochs. Indeed, several interesting proposals regarding various topics such as the generation of dark matter, matter-antimatter asymmetry, gravitational waves, primordial black holes, or microhalos during a nonstandard expansion phase have been recently made. In this paper, we review various possible causes and consequences of deviations from radiation domination in the early Universe - taking place either before or after BBN - and the constraints on them, as they have been discussed in the literature during the recent years.

astro-ph.CO↗

arXiv2021

Spectator dark matter in non-standard cosmologies

It has been shown that the observed dark matter (DM) abundance can be produced by amplification of quantum fluctuations of an energetically subdominant scalar field during inflation. In this paper, we study the robustness of this "spectator dark matter" scenario to changes in the expansion rate of the early Universe. Compared to the standard radiation-dominated (RD) scenario, two aspects will change: the DM energy density evolves differently as a function of time, and also the DM isocurvature perturbation spectrum will be different from the result in the RD case. These can impose sizeable changes to the values of model parameters which allow the field to constitute all DM while simultaneously satisfying all observational constraints. We study both free and self-interacting DM in scenarios with non-standard expansion and quantify the changes to the cases with a standard cosmological history. We also discuss testability of the scenario through primordial DM isocurvature and non-Gaussianity.

astro-ph.CO↗

arXiv2020

Gravitational wave constraints on the observable inflation

Gravitational waves (GW) produced in the early Universe contribute to the number of relativistic degrees of freedom, $N_{\rm eff}$, during Big Bang Nucleosynthesis (BBN). By using the constraints on $N_{\rm eff}$, we present a new bound on how much the Universe could have expanded between horizon exit of the largest observable scales today and the end of inflation. We discuss the implications on inflationary models and show how the new constraints affect model selection. We also discuss the sensitivities of the current and planned GW observatories such as LIGO and LISA, and show that the constraints they could impose are always less stringent than the BBN bound.

astro-ph.CO↗

arXiv2020

Violation of slow-roll in non-minimal inflation

We show that a non-minimal coupling to gravity can not only make some inflationary models consistent with cosmological data, similar to the case of Higgs inflation, but can also invoke slow-roll violation to realize graceful exit from inflation. In particular, this is the case in models where a destabilizing mechanism that ends inflation should be assumed when the model is minimally coupled to gravity. As explicit examples, we consider the power-law and inverse monomial inflation models with a non-minimal coupling to gravity. While these models are excluded in the minimally coupled case, we show that they can become viable again in non-minimally coupled scenarios. In most scenarios we considered, reheating can be naturally realized via gravitational particle production but that this depends on the underlying theory of gravity in a non-trivial way.

astro-ph.CO↗

arXiv2020

Initial conditions for plateau inflation: a case study

We study initial conditions for inflation in scenarios where the inflaton potential has a plateau shape. Such models are those most favored by Planck data and can be obtained in a large number of model classes. As a representative example, we consider Higgs inflation with and without an $R^2$ term in the context of Palatini gravity. We show that inflation with a large number of e-folds generically occurs in a large part of the parameter space without any fine-tuning of parameters even when the scale of inflation and the inflaton field value during inflation are much smaller than the Planck scale. We discuss consequences for detection of primordial gravitational waves and spectral tilt of curvature perturbations, as well as the recently proposed "Trans-Planckian Censorship" conjecture.

astro-ph.CO↗

arXiv2020

Tracing the high energy theory of gravity: an introduction to Palatini inflation

We present an introduction to cosmic inflation in the context of Palatini gravity, which is an interesting alternative to the usual metric theory of gravity. In the latter case only the metric $g_{μν}$ determines the geometry of space-time, whereas in the former case both the metric and the space-time connection $Γ^λ_{μν}$ are a priori independent variables - a choice which can lead to a theory of gravity different from the metric one. In scenarios where the field(s) responsible for cosmic inflation are coupled non-minimally to gravity or the gravitational sector is otherwise extended, assumptions of the underlying gravitational degrees of freedom can have a big impact on the observational consequences of inflation. We demonstrate this explicitly by reviewing several interesting and well-motivated scenarios including Higgs inflation, $R^2$ inflation, and $ξ$-attractor models. We also discuss some prospects for future research and argue why $r=10^{-3}$ is a particularly important goal for future missions that search for signatures of primordial gravitational waves.

astro-ph.CO↗

arXiv2020

Trans-Planckian Censorship, Inflation and Dark Matter

If the inflationary phase lasted longer than the minimal period, the length scales observed today originate from modes that were smaller than the Planck length during inflation. It was recently argued that this "trans-Planckian problem" can never arise in a consistent string theory framework, which places a stringent constraint on the energy scale of inflation, $V^{1/4}\lesssim 10^9$ GeV. In this paper, we show that this requirement corresponds to a very small Hubble scale during inflation, $H_{\rm inf}\lesssim 1$ GeV, and therefore has serious consequences on scenarios where the dark matter density was generated by amplification of quantum fluctuations during inflation. We also present a class of inflationary models which both satisfy the above limit for the scale of inflation and are in perfect agreement with observational data.

astro-ph.CO↗

arXiv2019

Dark matter from scalar field fluctuations

Dark matter (DM) may have its origin in a pre-Big Bang epoch, the cosmic inflation. Here, we consider for the first time a broad class of scenarios where a massive free scalar field unavoidably reaches an equilibrium between its classical and quantum dynamics in a characteristic time scale during inflation and sources the DM density. The study gives the abundance and perturbation spectrum of any DM component sourced by the scalar field. We show that this class of scenarios generically predicts enhanced structure formation, allowing one to test models where DM interacts with matter only gravitationally.

astro-ph.CO↗

arXiv2019

Standard model Higgs field and hidden sector cosmology

We consider scenarios where the inflaton field decays dominantly to a hidden dark matter (DM) sector. By studying the typical behavior of the Standard Model (SM) Higgs field during inflation, we derive a relation between the primordial tensor-to-scalar ratio $r$ and amplitude of the residual DM isocurvature perturbations $β$ which is typically generated if the DM is thermally decoupled from the SM sector. We consider different expansion histories and find that if the Universe was radiation- or matter-dominated after inflation, a future discovery of primordial DM isocurvature will rule out all simple scenarios of this type because generating observable $β$ from the Higgs is not possible without violating the bounds on $r$. Seen another way, the Higgs field is generically not a threat to models where both the inflaton and DM reside in a decoupled sector. However, this is not necessarily the case for an early kination-dominated epoch, as then the Higgs can source sizeable $β$. We also discuss why the Higgs cannot source the observed curvature perturbation at large scales in any of the above cases but how the field can still be the dominant source of curvature perturbations at small scales.

astro-ph.CO↗

arXiv2019

Axion dark matter from Higgs inflation with an intermediate $H_*$

In order to accommodate the QCD axion as the dark matter (DM) in a model in which the Peccei-Quinn (PQ) symmetry is broken before the end of inflation, a relatively low scale of inflation has to be invoked in order to avoid bounds from DM isocurvature fluctuations, $H_* \lesssim \mathcal{O}(10^9)\,$GeV. We construct a simple model in which the Standard Model Higgs field is non-minimally coupled to Palatini gravity and acts as the inflaton, leading to a scale of inflation $H_* \sim 10^8\,$GeV. When the energy scale at which the PQ symmetry breaks is much larger than the scale of inflation, we find that in this scenario the required axion mass for which the axion constitutes all DM is $m_0 \lesssim 0.05{\rm \,μeV}$ for a quartic Higgs self-coupling $λ_ϕ= 0.1$, which correspond to the PQ breaking scale $v_σ\gtrsim 10^{14}\,$GeV and tensor-to-scalar ratio $r \sim 10^{-12}$. Future experiments sensitive to the relevant QCD axion mass scale can therefore shed light on the physics of the Universe before the end of inflation.

astro-ph.CO↗

arXiv2019

Scalar correlation functions in de Sitter space from the stochastic spectral expansion

We consider light scalar fields during inflation and show how the stochastic spectral expansion method can be used to calculate two-point correlation functions of an arbitrary local function of the field in de Sitter space. In particular, we use this approach for a massive scalar field with quartic self-interactions to calculate the fluctuation spectrum of the density contrast and compare it to other approximations. We find that neither Gaussian nor linear approximations accurately reproduce the power spectrum, and in fact always overestimate it. For example, for a scalar field with only a quartic term in the potential, $V=λϕ^4/4$, we find a blue spectrum with spectral index $n-1=0.579\sqrtλ$.

gr-qc↗

arXiv2019

Towards distinguishing variants of non-minimal inflation

We study models of inflation where the scalar field $ϕ$ that drives inflation is coupled non-minimally to gravity via $ξϕ^2 R$, or where the gravity sector is enlarged by an $R^2$ term. We consider the original Higgs inflation, Starobinsky inflation, and two different versions of a scenario where the inflaton is a scalar field other than the Higgs, and discuss if they can be distinguished from each other by measuring the tensor-to-scalar ratio and runnings of the spectral index of primordial curvature perturbations, on top of the amplitude and spectral index of the perturbations. We consider both metric and Palatini theories of gravity, showing how detailed studies of non-minimally coupled models can help to identify the inflaton field and how they may provide for a way to also distinguish between different theories of gravity in the present context.

astro-ph.CO↗

arXiv2019

Primordial black hole constraints for extended mass functions

We revisit the cosmological and astrophysical constraints on the fraction of the dark matter in primordial black holes (PBHs) with an extended mass function. We consider a variety of mass functions, all of which are described by three parameters: a characteristic mass and width and a dark matter fraction. Various observations then impose constraints on the dark matter fraction as a function of the first two parameters. We show how these constraints relate to those for a monochromatic mass function, demonstrating that they usually become more stringent in the extended case than the monochromatic one. Considering only the well-established bounds, and neglecting the ones that depend on additional astrophysical assumptions, we find that there are three mass windows, around $4\times 10^{-17}M_\odot,$ $2\times 10^{-14}M_\odot$ and $25-100M_\odot$, where PBHs can constitute all dark matter. However, if one includes all the bounds, PBHs can only constitute of order $10\%$ of the dark matter.

astro-ph.CO↗

arXiv2019

Minimal Higgs inflation with an $R^2$ term in Palatini gravity

It has recently been suggested that the Standard Model Higgs boson could act as the inflaton while minimally coupled to gravity - given that the gravity sector is extended with an $αR^2$ term and the underlying theory of gravity is of Palatini, rather than metric, type. In this paper, we revisit the idea and correct some shortcomings in earlier studies. We find that in this setup the Higgs can indeed act as the inflaton and that the tree-level predictions of the model for the spectral index and the tensor-to-scalar ratio are $n_s\simeq 0.941$, $r\simeq 0.3/(1+10^{-8}α)$, respectively, for a typical number of e-folds, $N=50$, between horizon exit of the pivot scale $k=0.05\, {\rm Mpc}^{-1}$ and the end of inflation. Even though the tensor-to-scalar ratio is suppressed compared to the usual minimally coupled case and can be made compatible with data for large enough $α$, the result for $n_s$ is in severe tension with the Planck results. We briefly discuss extensions of the model.

astro-ph.CO↗

arXiv2019

Hidden Inflaton Dark Matter

If cosmic inflation was driven by an electrically neutral scalar field stable on cosmological time scales, the field necessarily constitutes all or part of dark matter (DM). We study this possibility in a scenario where the inflaton field $s$ resides in a hidden sector, which is coupled to the Standard Model sector through the Higgs portal $λ_{hs} s^2\mathcal{H}^\dagger\mathcal{H}$ and non-minimally to gravity via $ξ_s s^2 R$. We study scenarios where the field $s$ first drives inflation, then reheats the Universe, and later constitutes all DM. We consider two benchmark scenarios where the DM abundance is generated either by production during reheating or via non-thermal freeze-in. In both cases, we take into account all production channels relevant for DM in the mass range from keV to PeV scale. On the inflationary side, we compare the dynamics and the relevant observables in two different but well-motivated theories of gravity (metric and Palatini), discuss multifield effects in case both fields ($s$ and $h$) were dynamical during inflation, and take into account the non-perturbative nature of particle production during reheating. We find that, depending on the initial conditions for inflation, couplings and the DM mass, the scenario works well especially for large DM masses, $10^2$ GeV$\lesssim m_{s}\lesssim 10^6$ GeV, although there are also small observationally allowed windows at the keV and MeV scales. We discuss how the model can be tested through astrophysical observations.

hep-ph↗

arXiv2019

Phenomenology of Self-Interacting Dark Matter in a Matter-Dominated Universe

We study production of self-interacting dark matter (DM) during an early matter-dominated phase. As a benchmark scenario, we consider a model where the DM consists of singlet scalar particles coupled to the visible Standard Model (SM) sector via the Higgs portal. We consider scenarios where the initial DM abundance is set by either the usual thermal freeze-out or an alternative freeze-in mechanism, where DM was never in thermal equilibrium with the SM sector. For the first time, we take the effect of self-interactions within the hidden sector into account in determining the DM abundance, reminiscent to the Strongly Interacting Massive Particle (SIMP) scenario. In all cases, the number density of DM may change considerably compared to the standard radiation-dominated case, having important observational and experimental ramifications.

hep-ph↗

arXiv2019

Scalar singlet dark matter in non-standard cosmologies

We study production of dark matter (DM) in models with a non-standard expansion history. We consider both freeze-out and freeze-in mechanisms for producing the observed DM abundance in a model where the DM consists of scalar singlet particles coupled to the Standard Model sector via the Higgs portal. We show that a non-standard expansion phase can lead to a significant change in the DM abundance and therefore to observational ramifications. For example, for DM freeze-in the required portal coupling can be much larger, whereas for DM freeze-out much smaller values become allowed. We evaluate the relevant constraints and discuss prospects for direct detection of such DM.

hep-ph↗

arXiv2019

Spectator Dark Matter

The observed dark matter abundance in the Universe can be fully accounted for by a minimally coupled spectator scalar field that was light during inflation and has sufficiently strong self-coupling. In this scenario, dark matter was produced during inflation by amplification of quantum fluctuations of the spectator field. The self-interaction of the field suppresses its fluctuations on large scales, and therefore avoids isocurvature constraints. The scenario does not require any fine-tuning of parameters. In the simplest case of a single real scalar field, the mass of the dark matter particle would be in the range $1~{\rm GeV}\lesssim m\lesssim 10^8~{\rm GeV}$, depending on the scale of inflation, and the lower bound for the quartic self-coupling is $λ\gtrsim 0.45$.

astro-ph.CO↗