Baryon Inhomogeneities Research Articles

Small-scale clumping at recombination and the Hubble tension

Despite the success of the standard $\Lambda$CDM model of cosmology, recent data improvements have made tensions emerge between low- and high-redshift observables, most importantly in determinations of the Hubble constant $H_0$ and the (rescaled) clustering amplitude $S_8$. The high-redshift data, from the cosmic microwave background (CMB), crucially relies on recombination physics for its interpretation. Here we study how small-scale baryon inhomogeneities (i.e., clumping) can affect recombination and consider whether they can relieve both the $H_0$ and $S_8$ tensions. Such small-scale clumping, which may be caused by primordial magnetic fields or baryon isocurvature below kpc scales, enhances the recombination rate even when averaged over larger scales, shifting recombination to earlier times. We introduce a flexible clumping model, parametrized via three spatial zones with free densities and volume fractions, and use it to study the impact of clumping on CMB observables. We find that increasing $H_0$ decreases both $\Omega_m$ and $S_8$, which alleviates the $S_8$ tension. On the other hand, the shift in $\Omega_m$ is disfavored by the low-$z$ baryon-acoustic-oscillations measurements. We find that the clumping parameters that can change the CMB sound horizon enough to explain the $H_0$ tension also alter the damping tail, so they are disfavored by current Planck 2018 data. We test how the CMB damping-tail information rules out changes to recombination by first removing $\ell>1000$ multipoles in Planck data, where we find that clumping could resolve the $H_0$ tension. Furthermore, we make predictions for future CMB experiments, as their improved damping-tail precision can better constrain departures from standard recombination. Both the Simons Observatory and CMB-S4 will provide decisive evidence for or against clumping as a resolution to the $H_0$ tension.

Can small-scale baryon inhomogeneities resolve the Hubble tension? An investigation with ACT DR4

Small-scale inhomogeneities in the baryon density around recombination have been proposed as a solution to the tension between local and global determinations of the Hubble constant. These baryon clumping models make distinct predictions for the cosmic microwave background anisotropy power spectra on small angular scales. We use recent data from the Atacama Cosmology Telescope to test these predictions. No evidence for baryon clumping is found, assuming a range of parameterizations for time-independent baryon density probability distribution functions. The inferred Hubble constant remains in significant tension with the SH0ES measurement.

Decay of baryon inhomogeneities in an expanding universe

Baryon inhomogeneities are generated early in the universe. These inhomogeneities affect the phase transition dynamics of subsequent phase transitions, they also affect the nucleosynthesis calculations. We study the decay of the inhomogeneities in the early universe using the diffusion equation in the Friedmann–Lemaître–Robertson–Walker metric. We calculate the interaction cross section of the quarks with the neutrinos, the electrons and the muons and obtain the diffusion coefficients. The diffusion coefficients are temperature dependent. We find that the expansion of the universe causes the inhomogeneities to decay at a faster rate. We find that the baryon inhomogeneities generated at the electroweak epoch have low amplitudes at the time of the quark hadron transition and hence will not affect the phase transition dynamics unless they are generated with a amplitude greater than 10^{5} times the background density. After the quark hadron transition, we include the interaction of the muons with the hadrons till 100 MeV. We find that large density inhomogeneities generated during the quark hadron transition with sizes of the order of 1 km must have amplitudes greater than 10^{5} times the background density to survive upto the nucleosynthesis epoch. This puts constraints on any models that generate these inhomogeneities

Relieving the Hubble Tension with Primordial Magnetic Fields.

The standard cosmological model determined from the accurate cosmic microwave background measurements made by the Planck satellite implies a value of the Hubble constant H_{0} that is 4.2 standard deviations lower than the one determined from type Ia supernovae. The Planck best fit model also predicts higher values of the matter density fraction Ω_{m} and clustering amplitude S_{8} compared to those obtained from the Dark Energy Survey Year 1 data. Here we show that accounting for the enhanced recombination rate due to additional small-scale inhomogeneities in the baryon density may solve both the H_{0} and the S_{8}-Ω_{m} tensions. The additional baryon inhomogeneities can be induced by primordial magnetic fields present in the plasma prior to recombination. The required field strength to solve the Hubble tension is just what is needed to explain the existence of galactic, cluster, and extragalactic magnetic fields without relying on dynamo amplification. Our results show clear evidence for this effect and motivate further detailed studies of primordial magnetic fields, setting several well-defined targets for future observations.

FAST RADIO BURST/GAMMA-RAY BURST COSMOGRAPHY

Recently, both theoretical arguments and observational evidence suggested that a small fraction of fast radio bursts (FRBs) could be associated with gamma-ray bursts (GRBs). If such FRB/GRB association systems are commonly detected in the future, the combination of dispersion measure (DM) derived from FRBs and redshifts derived from GRBs makes these systems a plausible tool to conduct cosmography. We quantify uncertainties in deriving the redshift-dependent DM_{IGM} as a function of z, and test how well dark energy models can be constrained with Monte Carlo simulations. We show that with potentially several 10s of FRB/GRB systems detected in a decade or so, one may reach reasonable constraints on wCDM models. When combined with SN Ia data, unprecedented constraints on dark energy equation of state may be achieved, thanks to the prospects of detecting FRB/GRB systems at relatively high redshifts. The ratio between the mean value < DM_{IGM} (z)> and luminosity distance (D_{L} (z)) is insensitive to dark energy models. This gives the prospects of applying SN Ia data to calibrate < DM_{IGM} (z)> using a relatively small sample of FRB/GRB systems, allowing a reliable constraint on the baryon inhomogeneity distribution as a function of redshift. The methodology developed in this paper can also be applied, if the FRB redshifts can be measured by other means. Some caveats of putting this method into practice are also discussed.

Baryon inhomogeneities due to CP violating QCD Z(3) walls

We have investigated the possibility of generating baryon inhomogeneities in the quark-gluon plasma phase in the early universe due to CP violating scattering of quarks and anti-quarks from moving Z(3) domain walls. CP violation, here, is spontaneous in nature, and arises from the non-trivial profile of the background gauge field between different Z(3) vacua. We have calculated the spatial variation of the background gauge field across the Z(3) interface from the profile of the Polyakov loop L(x) for the Z(3) interface and determined the reflection of quarks and anti-quarks using the Dirac equation. Our results have shown that the reflection coefficients of quarks and anti-quarks can differ by a large amount. We have discussed the implications of this for baryon inhomogeneity generation from collapsing large Z(3) walls.

Baryon inhomogeneities in a charged quark gluon plasma

We study the generation of baryon inhomogeneities in regions of the quark gluon plasma which have a charge imbalance. We find that the overdensity in the baryon lumps for positively charged particles is different from the overdensity due to the negatively charged particles. Since quarks are charged particles, the probability of forming neutrons or protons in the lumps would thus be changed. The probability of forming hadrons having quarks of the same charges would be enhanced. This might have interesting consequences for the inhomogeneous nucleosynthesis calculations.

Baryon inhomogeneity generation in the quark-gluon plasma phase

We discuss the possibility of generation of baryon inhomogeneities in a quark-gluon plasma phase due to moving $Z(3)$ interfaces. By modeling the dependence of effective mass of the quarks on the Polyakov loop order parameter, we study the reflection of quarks from collapsing $Z(3)$ interfaces and estimate resulting baryon inhomogeneities in the context of the early universe. We argue that in the context of certain low energy scale inflationary models, it is possible that large $Z(3)$ walls arise at the end of the reheating stage. Collapse of such walls could lead to baryon inhomogeneities which may be separated by large distances near the QCD scale. Importantly, the generation of these inhomogeneities is insensitive to the order, or even the existence, of the quark-hadron phase transition. We also briefly discuss the possibility of formation of quark nuggets in this model, as well as baryon inhomogeneity generation in relativistic heavy-ion collisions.

Implications of cosmic string-induced density fluctuations at the quark-hadron transition

We show that cosmic strings moving through the plasma at the time of a first-order quark-hadron transition in the early universe generate baryon inhomogeneities, which can survive till the nucleosynthesis epoch. We find out how these inhomogeneities actually affect the calculated values of the light element abundances. Recently a wealth of observational data from various experiments have helped to reduce the uncertainties in the values of these abundances. Using these we show that it is possible to derive constraints in the presence of cosmic strings during the quark-hadron transition.

Effect of pre-existing baryon inhomogeneities on the dynamics of quark-hadron transition

Baryon number inhomogeneities may be generated during the epoch when the baryon asymmetry of the universe is produced, e.g. at the electroweak phase transition. These lumps will have a lower temperature than the background. Also the value ofT c will be different in these regions. Since a first-order quark-hadron (Q–H) transition is susceptible to small changes in temperature, we investigate the effect of the presence of such baryonic lumps on the dynamics of the Q–H transition. We find that the phase transition is delayed in these lumps for significant overdensities. Consequently, we argue that baryon concentration in these regions grows by the end of the transition. We mention some models which may give rise to such high baryon overdensities before the Q–H transition.

Baryon inhomogeneities due to cosmic string wakes at the quark-hadron transition

Baryon inhomogeneities generated during the quark-hadron transition may alter the abundances of light elements if they persist up to the time of nucleosynthesis. These inhomogeneities survive up to the nucleosynthesis epoch if they are separated by a distance of at least a few metres. In this work we present a model where large sheets of these inhomogeneities separated by a distance of a few km are formed by cosmic string wakes during the quark-hadron transition. The effect of these sheets on nucleosynthesis will also put constraints on the various cosmic string parameters.

Baryon inhomogeneity generation via cosmic strings at QCD scale and its effects on nucleosynthesis

We have earlier shown that cosmic strings moving through the plasma at the time of a first order quark-hadron transition in the early universe can generate large scale baryon inhomogeneities. In this paper, we calculate detailed structure of these inhomogeneities at the quark-hadron transition. Our calculations show that the inhomogeneities generated by cosmic string wakes can strongly affect nucleosynthesis calculations. A comparison with observational data suggests that such baryon inhomogeneities should not have existed at the nucleosynthesis epoch. If this disagreement holds with more accurate observations, then it will lead to the conclusions that cosmic string formation scales above ${10}^{14}--{10}^{15}$ GeV may not be consistent with nucleosynthesis and CMBR observations. Alternatively, some other input in our calculation should be constrained, for example, if the average string velocity remains sufficiently small so that significant density perturbations are never produced at the QCD scale, or if strings move ultrarelativistically so that string wakes are very thin, trapping negligible amount of baryons. Finally, if the quark-hadron transition is not of first order then our calculations do not apply.

Effect of pre-existing baryon inhomogeneities on the dynamics of the quark-hadron transition

Baryon number inhomogeneities may be generated during the epoch when the baryon asymmetry of the universe is produced, e.g. at the electroweak phase transition. The regions with excess baryon number will have a lower temperature than the background temperature of the universe. Also the value of the quark hadron transition temperature ${T}_{c}$ will be different in these regions as compared to the background region. Since a first-order quark hadron transition is very susceptible to small changes in temperature, we investigate the effect of the presence of such baryonic lumps on the dynamics of the quark-hadron transition. We find that the phase transition is delayed in these lumps for significant overdensities. Consequently, we argue that baryon concentration in these regions grows by the end of the transition. We briefly discuss some models which may give rise to such high overdensities at the onset of the quark-hadron transition.

Inhomogeneous big bang nucleosynthesis: Upper limit onΩband production of lithium, beryllium, and boron

We examine the big bang nucleosynthesis (BBN) process in the presence of small-scale baryon inhomogeneities. Primordial abundance yields for D, ${}^{4}\mathrm{He},$ ${}^{6}\mathrm{Li},$ ${}^{7}\mathrm{Li},$ ${}^{9}\mathrm{Be},$ and ${}^{11}\mathrm{B}$ are computed for wide ranges of parameters characterizing the inhomogeneities taking account of all relevant diffusive and hydrodynamic processes. These calculations may be of interest due to (a) recent observations of the anisotropies in the cosmic microwave background radiation favoring slightly larger baryonic contribution to the critical density, ${\ensuremath{\Omega}}_{b},$ than allowed by a standard BBN scenario and (b) new observational determinations of ${}^{6}\mathrm{Li}$ and ${}^{9}\mathrm{Be}$ in metal-poor halo stars. We find considerable parameter space in which production of D and ${}^{4}\mathrm{He}$ is in agreement with observational constraints even for ${\ensuremath{\Omega}}_{b}{h}^{2}$ a factor 2 or 3 larger than the ${\ensuremath{\Omega}}_{b}$ inferred from standard BBN. Nevertheless, in this parameter space synthesis of ${}^{7}\mathrm{Li}$ in excess of the inferred ${}^{7}\mathrm{Li}$ abundance on the Spite plateau results. Production of ${}^{6}\mathrm{Li},$ ${}^{9}\mathrm{Be},$ and ${}^{11}\mathrm{B}$ in inhomogeneous BBN scenarios is still typically well below the abundance of these isotopes observed in the most metal-poor stars to date thus neither confirming nor rejecting inhomogeneous BBN. In an Appendix we summarize results of a reevaluation of baryon diffusion constants entering inhomogeneous BBN calculations.

Cosmic string induced sheetlike baryon inhomogeneities at the quark-hadron transition

Cosmic strings moving through matter produce wakes where the density is higher than the background density. We investigate the effects of such wakes occurring at the time of a first order quark-hadron transition in the early universe and show that they can lead to a separation of the quark-gluon plasma phase in the wake region, while the region outside the wake converts to the hadronic phase. Moving interfaces then trap large baryon densities in sheetlike regions which can extend across the entire horizon. A typical separation between such sheets, at formation, is of the order of 1 km. Regions of baryon inhomogeneity of this nature, i.e., having a planar geometry and separated by such large distance scales, appear to be well suited for recent models of inhomogeneous nucleosynthesis to reconcile the large baryon to photon ratio implied by the recent measurements of the cosmic microwave background power spectrum.

QCD phase transition in the inhomogeneous universe.

We investigate a new mechanism for the cosmological QCD phase transition: inhomogeneous nucleation. The primordial temperature fluctuations, measured to be deltaT/T approximately 10(-5), are larger than the tiny temperature interval in which bubbles would form in the standard picture of homogeneous nucleation. Thus the bubbles nucleate at cold spots. We find the typical distance between bubble centers to be a few meters. This exceeds the estimates from homogeneous nucleation by 2 orders of magnitude. The resulting baryon inhomogeneities may affect primordial nucleosynthesis.

Relics of the cosmological QCD phase transition

The abundance and size distribution of quark nuggets (QN), formed a few microseconds after the big bang due to first order QCD phase transition in the early universe, has been estimated. It appears that stable QNs could be a viable candidate for cosmological dark matter. The evolution of baryon inhomogeneity due to evaporated (unstable) QNs are also examined.

Inhomogeneous big-bang nucleosynthesis in light of recent observations

We consider inhomogeneous big-bang nucleosynthesis in light of the present observational situation. We give updated limits to the baryon-to-photon ratio and ${\ensuremath{\Omega}}_{b}$ and show that the inhomogeneous nucleosynthesis may alleviate the tension between ${}^{4}\mathrm{He}$ and D observations. The recent Kamiokande results are shown to strongly disfavor the decaying $\ensuremath{\tau}$ neutrino solution to this tension. The possible sources of baryon inhomogeneity include the QCD and electroweak phase transitions (EWPT). It is found that EWPT produces a distinct ``beer foam'' geometry of inhomogeneity, but that the distance scale is too small for them to have a large effect on nucleosynthesis; the effect may still be larger than some of the other small corrections recently incorporated to SBBN codes.

Inhomogeneity scale from the cosmological quark - hadron transition

The mean nucleation distance, , is an important parameter in a first-order cosmological quark - hadron phase transition, as baryon inhomogeneities generated during the transition may persist to the time of nucleosynthesis, and alter the outcome of the light elements, provided is above 1 m. We describe different scenarios based on the MIT bag model to determine this distance, and briefly mention ongoing work that may completely change our picture of the phase transition.

Effect of strong magnetic field on cosmic quark-hadron phase transition and baryon inhomogeneity

The effect of intense magnetic field on the cosmic quark-hadron phase transition and also on the baryon number inhomogeneity has been investigated using phenomenological MIT bag model for the quark sector. For the sake of simplicity an ideal gas equation of state has been considered for the hadronic phase.