Inflationary Gravitational Waves Research Articles

Constraining the general oscillatory inflaton potential with freeze-indark matter and gravitational waves

The reheating phase after inflation is one of the least observationally constrained epochs in the evolution of the Universe. The forthcoming gravitational wave observatories will enable us to constrain at least some of the non-standard scenarios. For example, models where the radiation bath is produced by the perturbative inflaton decay that oscillates around a minimum of the potential of the form V ∝ ϕ 2n, with n > 2. In such scenarios a part of the inflationary gravitational wave spectrum becomes blue tilted, making it observable, depending on the inflation energy scale and the reheating temperature. The degeneracy between the latter two parameters can be broken if dark matter in the Universe is produced via the freeze-in mechanism. The combination of the independent measurement of dark matter mass with gravitational wave observations makes it possible to constrain the reheating temperature and the energy scale at the end of inflation, at least within some parameter ranges.

LiteBIRD science goals and forecasts: improving sensitivity to inflationary gravitational waves with multitracer delensing

We estimate the efficiency of mitigating the lensing B-mode polarization, the so-called delensing, for the LiteBIRD experiment with multiple external data sets of lensing-mass tracers.The current best bound on the tensor-to-scalar ratio, r, is limited by lensing rather than Galactic foregrounds.Delensing will be a critical step to improve sensitivity to r as measurements of r become more and more limited by lensing.In this paper, we extend the analysis of the recent LiteBIRD forecast paper to include multiple mass tracers, i.e., the CMB lensing maps from LiteBIRD and CMB-S4-like experiment, cosmic infrared background, and galaxy number density from Euclid- and LSST-like survey.We find that multi-tracer delensing will further improve the constraint on r by about 20%. In LiteBIRD, the residual Galactic foregrounds also significantly contribute to uncertainties of the B-modes, and delensing becomes more important if the residual foregrounds are further reduced by an improved component separation method.

Broken blue-tilted inflationary gravitational waves: a joint analysis of NANOGrav 15-year and BICEP/Keck 2018 data

Recently, the pulsar timing array (PTA) collaborations havereported the evidence for a stochastic gravitational wavebackground (SGWB) at nano-Hertz band. The spectrum of inflationarygravitational wave (IGW) is unknown, which might exhibit differentpower law at different frequency-bands, thus if the PTA signal isprimordial, it will be significant to explore the underlyingimplications of current PTA and CMB data on IGW. In this Letter,we perform a joint Markov Chain Monte Carlo analysis for a brokenpower-law spectrum of IGW with the NANOGrav 15-year and BICEP/Keck2018 data. It is found that though the bestfit spectral tilt ofIGW at PTA band is n PTA T = 2.42+0.32 -0.91,at CMB band the bestfit is n CMB T = 0.55+0.37 -0.10 while a detectable amplitude of r with n CMB T ≃ 0 is still compatible. Theimplication of our results for inflation is also discussed.

Imprint of inflationary gravitational waves and WIMP dark matter in pulsar timing array data

Motivated by the recent release of new results from five different pulsar timing array (PTA) experiments claiming to have found compelling evidence for primordial gravitational waves (GW) at nano-Hz frequencies, we consider the prospects of generating such a signal from inflationary blue-tilted tensor power spectrum in a specific dark matter (DM) scenario dubbed as Miracle-less WIMP. While Miracle-less WIMP, due to insufficient interaction rate with the Standard Model (SM) bath gets thermally overproduced, inflationary blue-tilted gravitational waves (BGW) in compliance with PTA data, conflict cosmological observations if reheat temperature after inflation is sufficiently high. Both these issues are circumvented with late entropy dilution, bringing DM abundance within observational limits and creating a doubly-peaked feature in the BGW spectrum consistent with cosmological observations. The blue-tilted tail of the low-frequency peak can fit NANOGrav 15 yr data, while other parts of the spectrum are within reach of present and future GW experiments.

Fingerprints of GeV scale right-handed neutrinos on inflationary gravitational waves and PTA data

Fingerprints of GeV scale right-handed neutrinos on inflationary gravitational waves and PTA data

Inflationary gravitational wave background as a tail effect

Inflationary gravitational wave background as a tail effect

New Probe of Inflationary Gravitational Waves: Cross-Correlations of Lensed Primary CMB B-Modes with Large-Scale Structure.

We propose a new probe of inflationary gravitational waves (IGWs): the cross-correlation of the lensing of inflationary B-mode polarization with a large-scale structure (LSS) tracer, which can also be a cosmic microwave background (CMB) lensing map. This is equivalent to measuring a three-point function of two CMB B-modes and an LSS tracer. We forecast expected 1σ constraints on the tensor-to-scalar ratio r, albeit with a simplistic foreground treatment, and find constraints of σ_{r}≃7×10^{-3} from the correlation of CMB-S4-Deep B-mode lensing and LSST galaxies, σ_{r}≃5×10^{-3} from the correlation of CMB-S4-Deep B-mode lensing and CMB-S4-Deep CMB lensing, and σ_{r}≃10^{-2} from the correlation of LiteBIRD B-mode lensing and CMB-S4-Wide lensing. Because this probe is inherently non-Gaussian, simple Gaussian foregrounds will not produce any biases to the measurement of r. While a detailed investigation of non-Gaussian foreground contamination for different cross-correlations will be essential, this observable has the potential to be a useful probe of IGWs, which, due to different sensitivity to many potential sources of systematic errors, can be complementary to standard methods for constraining r.

Tensor-to-scalar ratio forecasts for extended LiteBIRD frequency configurations

LiteBIRD is a planned JAXA-led cosmic microwave background (CMB) B-mode satellite experiment aiming for launch in the late 2020s, with a primary goal of detecting the imprint of primordial inflationary gravitational waves. Its current baseline focal-plane configuration includes 15 frequency bands between 40 and 402 GHz, fulfilling the mission requirements to detect the amplitude of gravitational waves with the total uncertainty on the tensor-to-scalar ratio, δr, down to δr < 0.001. A key aspect of this performance is accurate astrophysical component separation, and the ability to remove polarized thermal dust emission is particularly important. In this paper we note that the CMB frequency spectrum falls off nearly exponentially above 300 GHz relative to the thermal dust spectral energy distribution, and a relatively minor high frequency extension can therefore result in even lower uncertainties and better model reconstructions. Specifically, we compared the baseline design with five extended configurations, while varying the underlying dust modeling, in each of which the High-Frequency Telescope (HFT) frequency range was shifted logarithmically toward higher frequencies, with an upper cutoff ranging between 400 and 600 GHz. In each case, we measured the tensor-to-scalar ratio r uncertainty and bias using both parametric and minimum-variance component-separation algorithms. When the thermal dust sky model includes a spatially varying spectral index and temperature, we find that the statistical uncertainty on r after foreground cleaning may be reduced by as much as 30–50% by extending the upper limit of the frequency range from 400 to 600 GHz, with most of the improvement already gained at 500 GHz. We also note that a broader frequency range leads to higher residuals when fitting an incorrect dust model, but also it is easier to discriminate between models through higher χ2 sensitivity. Even in the case in which the fitting procedure does not correspond to the underlying dust model in the sky, and when the highest frequency data cannot be modeled with sufficient fidelity and must be excluded from the analysis, the uncertainty on r increases by only about 5% for a 500 GHz configuration compared to the baseline.

Revisiting the cosmological phase transitions on the inflationary gravitational waves with BKP18

The effects of the electroweak and the quark-gluon plasma phase transitions are considered in the course of evolution of the universe and the resulting spectrum of primordial gravitational waves normalized with the BICEP/Keck and Planck joint data is obtained. The QGP index n=0.203 leads to a step reduction of ∼17.6% in the amplitude and a step of ∼35% in the energy density spectrum for frequency >1.36×10−8Hz. On the other hand, the EW index w=0.0147 leads to a step of ∼1.47% in the amplitude and a step of ∼2.9% in the energy density spectrum for frequency >10−5Hz. The combined effect of phase transitions on the inflationary gravitational waves leads to about ∼19% reduction in the amplitude and an overall reduction of ∼38% in the energy density as compared to the spectrum without considering these phase transitions. The resulting spectrum for R2 inflation model is found to be well within the BBN bound and its potential detectability is tested with the power-law integrated sensitivity curves of the future LISA, DECIGO, BBO, SKA and IPTA experiments.

Impact of high-scale Seesaw and Leptogenesis on inflationary tensor perturbations as detectable gravitational waves

We discuss the damping of inflationary gravitational waves (GW) that re-enter the horizon before or during an epoch, where the energy budget of the universe is dominated by an unstable right handed neutrino (RHN), whose out of equilibrium decay releases entropy. Starting from the minimal Standard Model extension, motivated by the observed neutrino mass scale, with nothing more than 3 RHN for the Seesaw mechanism, we discuss the conditions for high scale leptogenesis assuming a thermal initial population of RHN. We further address the associated production of potentially light non-thermal dark matter and a potential component of dark radiation from the same RHN decay. One of our main findings is that the frequency, above which the damping of the tensor modes is potentially observable, is completely determined by successful leptogenesis and a Davidson-Ibarra type bound to be at around 0.1 Hz. To quantify the detection prospects of this GW background for various proposed interferometers such as AEDGE, BBO, DECIGO, Einstein Telescope or LISA we compute the signal-to-noise ratio (SNR). This allows us to investigate the viable parameter space of our model, spanned by the mass of the decaying RHN {M}_1gtrsim 2.4times {10}^8textrm{GeV}cdot sqrt{2times {10}^{-7}textrm{eV}/{tilde{m}}_1} (for leptogenesis) and the effective neutrino mass parameterizing its decay width {tilde{m}}_1 < 2.9 × 10−7 eV (for RHN matter domination). Thus gravitational wave astronomy is a novel way to probe both the Seesaw and the leptogenesis scale, which are completely inaccessible to laboratory experiments in high scale scenarios.

Sensing quantum nature of primordial gravitational waves using electromagnetic probes

Based on optical medium analogy, we establish a formalism to describe the interaction between an electromagnetic (EM) system with gravitational waves (GWs) background. After a full discussion on the classical treatment of the EM-GW interaction and finding the EM field mode-functions in the presence of the magneto-dielectric media caused by GWs, the governing quantum interaction Hamiltonian is obtained. Investigation of the optical quadrature variance as well as the visibility of a laser field interacting with the multi-mode squeezed primordial gravitational waves imply that the inflationary primordial gravitational waves (PGWs) act as a decoherence mechanism that destroy EM coherency after a characteristic time scale, τ c , which depends on the inflationary parameters (β, β s , r), or equivalently, the fractional energy density of PGWs, Ω gw,0. The decoherency mechanism overcomes the coherent effects, such as revivals of optical squeezing, thus leaving their confirmation out of reach. Influenced by the continuum of the squeezed PGWs, the laser field suffers a line-width broadening by . The most peculiar property of the EM spectrum is the apparition of side bands at Hz, stemming from the squeezed nature of PGWs. The laser phase noise induced by the squeezed PGWs grows with time squarely, , that can most possibly be sensed within a finite flight time.

Updated constraints on amplitude and tilt of the tensor primordial spectrum

We have taken a comprehensive approach to update the limits on the tensor-to-scalar ratio (r) and the tensor spectral index (nt ), using 10 datasets from the BICEP/Keck Array 2015 and 2018, Planck releases 3 and 4, and LIGO-Virgo-KAGRA Collaboration. By fitting the complete ΛCDM+r+nt model with two different approaches for the tensor sector, we have not only established which method is the most reliable, but have also achieved the strongest constraint on the tensor-to-scalar ratio in current literature: r < 0.028 and -1.37 < nt < 0.42 at 95% confidence level. Furthermore, our examination of the common signal detected by the NANOGrav Collaboration further confirms that a simple power-law cannot reconcile the constraints from different datasets if the NANOGrav detection is due to a primordial inflationary gravitational wave background, as previously shown in the literature.

Generating enhanced parity-violating gravitational waves during inflation with violation of the null energy condition

A violation of the null energy condition (NEC) during inflation in a single-field inflation model will naturally enhance the amplitude of the parity violation effect (defined by $\Delta\chi$) of inflationary primordial gravitational waves (GWs), provided the inflaton is non-minimally coupled to a gravitational Chern-Simons term. After going through the NEC-violating phase, the Universe enters subsequent slow-roll inflation with a higher energy scale (i.e., a greater Hubble parameter $H$), which results in an enhanced nearly scale-invariant power spectrum (i.e., $P_{\rm T}$) of inflationary primordial GWs in the high-frequency band, while $P_{\rm T}$ remains consistent with observations in the frequency band of the cosmic microwave background. Therefore, the violation of NEC during inflation will amplify the observability (i.e., $P_{\rm T}\cdot\Delta\chi$) of the parity violation effect on small scales. Intriguingly, our model has particular oscillatory features on $\Delta\chi$ that may not be mimicked by others.

Extended delta-map: A map-based foreground removal method for CMB polarization observations

AbstractIn order to extract information about inflationary gravitational waves using B-mode patterns of cosmic microwave polarization anisotropy, we need to remove the foreground radiation from the Milky Way. In our previous delta-map method for foreground removal, the number of observation bands was limited to the number of parameters of the assumed foreground model, and therefore it was difficult to improve the sensitivity by increasing the number of observation bands. Here, we extend the previous method so that it can be adapted to an arbitrary number of observation bands. Using parametric likelihood and realistic foreground and CMB simulations, we show that our method can increase the sensitivity to the tensor-to-scalar ratio r without inducing any significant bias.

Probing parity-odd bispectra with anisotropies of GW V modes

It is well known that non-trivial squeezed tensor bispectra can lead to anisotropies in the inflationary stochastic gravitational wave (GW) background, providing us with an alternative and complementary window to primordial non-Gaussianities (NGs) with respect to the CMB. Previous works have highlighted the detection prospects of parity-even tensor NGs via the GW I-mode anisotropies. In this work we extend this by analysing for the first time the additional information carried by GW V-mode anisotropies due to squeezed NGs. We show that GW V modes allow us to probe parity-odd squeezed 〈 tts 〉 and 〈 ttt 〉 bispectra. These bispectra break parity at the non-linear level and can be introduced by allowing alternative symmetry breaking patterns during inflation, like those comprised in solid inflation. Considering a BBO-like experiment, we find that a non-zero detection of squeezed 〈 tts 〉 parity-odd bispectra in the V modes dipole is possible without requiring any short-scale enhancement of the GW power spectrum amplitude over the constraints set by the CMB. We also briefly discuss the role of V-CMB cross-correlations. Our work can be extended in several directions and motivates a systematic search for polarized GW anisotropies in the next generations of GW experiments.

E^+e^- annihilation on the stochastic background of primordial gravitational waves with BKP18

The evolution of inflationary primordial gravitational waves with the expansion of the universe is studied while taking into account the e^+e^- annihilation which leads to step correction in the spectrum of primordial gravitational waves which is sim 10% in the amplitude and sim 25% in the energy density. The amplitudes and energy densities are studied for the alpha -attractor inflation models which are in agreement with the recent BICEP/Keck and Planck 2018 joint result. The energy densities are well within the big bang nucleosynthesis bound, but the spectra for these models are very much below the detection range of the currently operating and upcoming detectors.

Novel cosmological tests from combining galaxy lensing and the polarized Sunyaev-Zel’dovich effect

The polarized Sunyaev-Zel'dovich (pSZ) effect is sourced by the Thomson scattering of CMB photons from distant free electrons and yields a novel view of the CMB quadrupole throughout the observable Universe. Galaxy shear measures the shape distortions of galaxies, probing both their local environment and the intervening matter distribution. Both observables give interesting constraints on the cosmological model; here we ask: what can be learnt from their combination? The pSZ-shear cross-spectrum measures the shear-galaxy-polarization bispectrum and contains contributions from (1) the Sachs-Wolfe (SW) effect, (2) the integrated Sachs-Wolfe (ISW) effect, and (3) inflationary gravitational waves. Since the modes contributing to the pSZ signal are not restricted to the Earth's past lightcone, the low-redshift cross-spectra could provide a novel constraint on dark energy via the ISW effect, whilst the SW signal couples scalar modes at very different times but at similar positions; this provides a unique probe of the Universe's homogeneous time evolution. We give expressions for all major contributions to the shear, galaxy, and pSZ auto- and cross-spectra, and evaluate their detectability via Fisher forecasts. Despite significant theoretical utility, the cross-spectra will be challenging to detect: combining CMB-S4 with Rubin yields a $1.6\sigma$ detection of the ISW contribution, which increases to $5.2\sigma$ for a futuristic experiment involving CMB-HD and a higher galaxy sample density. For parity-even (parity-odd) tensors, we predict a $1\sigma$ limit of $\sigma(r) = 0.9$ ($0.2$) for CMB-S4 and Rubin, or $0.3$ ($0.06$) for the more futuristic setup. Whilst this is significantly better than the constraints from galaxy shear alone (and less sensitive to systematics), it is unlikely to be competitive, but may serve as a useful cross-check.

Gravitational wave and CMB probes of axion kination

Rotations of an axion field in field space provide a natural origin for an era of kination domination, where the energy density is dominated by the kinetic term of the axion field, preceded by an early era of matter domination. Remarkably, no entropy is produced at the end of matter domination and hence these eras of matter and kination domination may occur even after Big Bang Nucleosynthesis. We derive constraints on these eras from both the cosmic microwave background and Big Bang Nucleosynthesis. We investigate how this cosmological scenario affects the spectrum of possible primordial gravitational waves and find that the spectrum features a triangular peak. We discuss how future observations of gravitational waves can probe the viable parameter space, including regions that produce axion dark matter by the kinetic misalignment mechanism or the baryon asymmetry by axiogenesis. For QCD axion dark matter produced by the kinetic misalignment mechanism, a modification to the inflationary gravitational wave spectrum occurs above 0.01 Hz and, for high values of the energy scale of inflation, the prospects for discovery are good. We briefly comment on implications for structure formation of the universe.

B-mode forecast of CMB-Bhārat

ABSTRACT Exploring Cosmic History and Origin (ECHO), popularly known as ‘CMB-Bh$\overline{a}$rat’, is a space mission that has been proposed to the Indian Space Research Organisation for the scientific exploitation of the cosmic microwave background (CMB) at the next level of precision and accuracy. The quest for the CMB polarization B-mode signals, generated by inflationary gravitational waves in the very early universe, is one of the key scientific goals of its experimental design. This work studies the potential of the proposed ECHO instrumental configuration to detect the target tensor-to-scalar ratio r ∼ 10−3 at 3σ significance level, which covers the predictions of a large class of inflationary models. We investigate the performance of two different component separation pipelines, ${\mathtt {NILC}}$ and ${\mathtt {Commander}}$, for the measurement of r in the presence of different physically motivated models of astrophysical foregrounds. For a simplistic foreground model (only polarized dust and synchrotron), both component separation pipelines can achieve the desired sensitivity of ECHO, i.e. σ(r = 0) ∼ (0.4–0.7) × 10−3. ${\mathtt {NILC}}$ performs better than ${\mathtt {Commander}}$ in terms of bias on recovered r for complex spectral models (power law and curved power law) of the synchrotron emission and complex dust models (dust decorrelation). Assuming 84 per cent delensing, we can achieve an improvement of σ(r = 0) by approximately 50 per cent as compared to the results obtained for the same configuration without any lensing correction.

Simons Observatory: Constraining inflationary gravitational waves with multitracer B -mode delensing

We introduce and validate a delensing framework for the Simons Observatory (SO), which will be used to improve constraints on inflationary gravitational waves (IGWs) by reducing the lensing noise in measurements of the $B$-modes in CMB polarization. SO will initially observe CMB by using three small aperture telescopes and one large-aperture telescope. While polarization maps from small-aperture telescopes will be used to constrain IGWs, the internal CMB lensing maps used to delens will be reconstructed from data from the large-aperture telescope. Since lensing maps obtained from the SO data will be noise-dominated on sub-degree scales, the SO lensing framework constructs a template for lensing-induced $B$-modes by combining internal CMB lensing maps with maps of the cosmic infrared background from Planck as well as galaxy density maps from the LSST survey. We construct a likelihood for constraining the tensor-to-scalar ratio $r$ that contains auto- and cross-spectra between observed $B$-modes and the lensing $B$-mode template. We test our delensing analysis pipeline on map-based simulations containing survey non-idealities, but that, for this initial exploration, does not include contamination from Galactic and extragalactic foregrounds. We find that the SO survey masking and inhomogeneous and atmospheric noise have very little impact on the delensing performance, and the $r$ constraint becomes $\sigma(r)\approx 0.0015$ which is close to that obtained from the idealized forecasts in the absence of the Galactic foreground and is nearly a factor of two tighter than without delensing. We also find that uncertainties in the external large-scale structure tracers used in our multi-tracer delensing pipeline lead to bias much smaller than the $1\,\sigma$ statistical uncertainties.