Search results for "REIONIZATION"
showing 10 items of 16 documents
CMB anisotropies: cosmic confusion and polarization
2001
Abstract Some physical effects producing Cosmic Microwave Background (CMB) anisotropies are briefly described. The CMB angular power spectrum is calculated -in appropriate cases- with the essential aim of estimating and comparing the effects produced by reionization and gravitational waves; thus a problem of Cosmic Confusion is pointed out. Accurate measurements of the CMB polarization could solve this problem in future. Some comments about the PLANCK mission —ESA project for anisotropy detection— are given.
Detection of GRB 060927 at z = 5.47: Implications for the Use of Gamma-Ray Bursts as Probes of the End of the Dark Ages
2007
We report on follow-up observations of the GRB 060927 using the ROTSE-IIIa telescope and a suite of larger aperture ground-based telescopes. An optical afterglow was detected 20 s after the burst, the earliest rest-frame detection of optical emission from any GRB. Spectroscopy performed with the VLT about 13 hours after the trigger shows a continuum break at lambda ~ 8070 A produced by neutral hydrogen absorption at z~5.6. We also detect an absorption line at 8158 A which we interpret as SiII at z=5.467. Hence, GRB 060927 is the second most distant GRB with a spectroscopically measured redshift. The shape of the red wing of the spectral break can be fitted by a damped Lyalpha profile with a…
Constraints on dark matter annihilation from CMB observations before Planck
2013
We compute the bounds on the dark matter (DM) annihilation cross section using the most recent Cosmic Microwave Background measurements from WMAP9, SPT'11 and ACT'10. We consider DM with mass in the MeV-TeV range annihilating 100% into either an e(+)e(-) or a mu(+)mu(-) pair. We consider a realistic energy deposition model, which includes the dependence on the redshift, DM mass and annihilation channel. We exclude the canonical thermal relic abundance cross section ( = 3 x 10(-26) cm(3)s(-1)) for DM masses below 30 GeV and 15 GeV for the e(+)e(-) and mu(+)mu(-) channels, respectively. A priori, DM annihilating in halos could also modify the reionization history of the Universe at late times…
EDGES result versus CMB and low-redshift constraints on ionization histories
2018
We examine the results from the Experiment to Detect the Global Epoch of Reionization Signature (EDGES), which has recently claimed the detection of a strong absorption in the 21 cm hyperfine transition line of neutral hydrogen, at redshifts demarcating the early stages of star formation. More concretely, we study the compatibility of the shape of the EDGES absorption profile, centered at a redshift of $z \sim 17.2$, with measurements of the reionization optical depth, the Gunn-Peterson optical depth, and Lyman-$\alpha$ emission from star-forming galaxies, for a variety of possible reionization models within the standard $\Lambda$CDM framework (that is, a Universe with a cosmological consta…
Warm dark matter and the ionization history of the Universe
2017
In warm dark matter scenarios structure formation is suppressed on small scales with respect to the cold dark matter case, reducing the number of low-mass halos and the fraction of ionized gas at high redshifts and thus, delaying reionization. This has an impact on the ionization history of the Universe and measurements of the optical depth to reionization, of the evolution of the global fraction of ionized gas and of the thermal history of the intergalactic medium, can be used to set constraints on the mass of the dark matter particle. However, the suppression of the fraction of ionized medium in these scenarios can be partly compensated by varying other parameters, as the ionization effic…
Harrison-Zel'dovich primordial spectrum is consistent with observations
2010
Inflation predicts primordial scalar perturbations with a nearly scale-invariant spectrum and a spectral index approximately unity (the Harrison--Zel'dovich (HZ) spectrum). The first important step for inflationary cosmology is to check the consistency of the HZ primordial spectrum with current observations. Recent analyses have claimed that a HZ primordial spectrum is excluded at more than 99% c.l.. Here we show that the HZ spectrum is only marginally disfavored if one considers a more general reionization scenario. Data from the Planck mission will settle the issue.
Constraining the primordial black hole abundance with 21-cm cosmology
2019
The discoveries of a number of binary black hole mergers by LIGO and VIRGO has reinvigorated the interest that primordial black holes (PBHs) of tens of solar masses could contribute non-negligibly to the dark matter energy density. Should even a small population of PBHs with masses $\gtrsim \mathcal{O}(M_\odot)$ exist, they could profoundly impact the properties of the intergalactic medium and provide insight into novel processes at work in the early Universe. We demonstrate here that observations of the 21cm transition in neutral hydrogen during the epochs of reionization and cosmic dawn will likely provide one of the most stringent tests of solar mass PBHs. In the context of 21cm cosmolog…
Late decaying 2-component dark matter scenario as an explanation of the AMS-02 positron excess
2016
The long standing anomaly in the positron flux as measured by the PAMELA and AMS-02 experiments could potentially be explained by dark matter (DM) annihilations. This scenario typically requires a large "boost factor" to be consistent with a thermal relic dark matter candidate produced via freeze-out. However, such an explanation is disfavored by constraints from CMB observations on energy deposition during the epoch of recombination. We discuss a scenario called late-decaying two-component dark matter (LD2DM), where the entire DM consists of two semi-degenerate species. Within this framework, the heavier species is produced as a thermal relic in the early universe and decays to the lighter…
Was there an early reionization component in our universe?
2017
A deep understanding of the Epoch of Reionization is still missing in our knowledge of the universe. While future probes will allow us to test the precise evolution of the free electron fraction from redshifts between $z\simeq 6$ and $z\simeq 20$, at present one could ask what kind of reionization processes are allowed by present Cosmic Microwave Background temperature and polarization measurements. An early contribution to reionization could imply a departure from the standard picture where star formation determines the reionization onset. BBy considering a broad class of possible reionization parameterizations, we find that current data do not require an early reionization component in ou…
Laser-stimulated two-step recombination of highly charged ions and electrons in a storage ring
1994
Two-step resonant laser-stimulated recombination of highly charged ions was performed for the first time. Nd:YAG laser pulses overlapped with an Ar[sup 18+] beam in the electron cooler of the ESR storage ring at GSI induced transitions from the continuum to the [ital n]=81 state of hydrogenlike Ar[sup 17+]. To avoid reionization in the bending magnet before reaching the detector, the [ital n]=81 population was transferred to a state well below the reionization threshold by a Ti:sapphire laser. Tuning of this laser yielded the [ital n]=81 to 36 and 37 transition-line profiles. The two-step method provides access to detailed Rydberg spectroscopy in an electron beam environment.