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Astrophysical phenomena such as exploding primordial black holes (PBHs), gamma-ray bursts (GRBs), compact object mergers, and supernovae, are expected to produce a single pulse of electromagnetic radiation detectable at the low-frequency end of the radio spectrum. Detection of any of these pulses would be significant for the study of the objects themselves, their host environments, and the interstellar/intergalactic medium. Furthermore, a detection of a radio transient from an exploding PBH could be a signature of an extra spatial dimension, which would drastically alter our perception of spacetime. However, even upper limits on the existence of PBHs, from transient searches, would be important to discussions of cosmology. We describe a method to carry out an agnostic single-dispersed-pulse search, and apply it to data collected with the Eight-meter-wavelength Transient Array (ETA). ETA is a radio telescope dedicated to searching for transient pulses. It consists of 12 crossed-dipole antenna stands with Galactic-noise-limited performance from 29-47 MHz. There is a vast amount of data collected from an ETA observation. It is therefore greatly beneficial to use a computer cluster, which works in parallel on different parts of a data set, in order to carry out a single-pulse search quickly and efficiently. Each spectrogram in a data set needs to be analyzed individually, without reference to the rest, in order to utilize a computer cluster's capabilities. The data reduction software has been developed for single-dispersed-pulse searches, and is described in this thesis. The data reduction involves sweeping through the collected data with a dedispersion routine assuming a range of dispersion measures. The resulting time series are searched with multiple matched filters for signals above a signal-to-noise threshold. Applying the single pulse search to ~ 30 hours of ETA data did not yield a compelling detection of an astrophysical signal. However, from ≈ 5 hours of interference-free data we find an observational upper limit to the rate of exploding PBHs of r ≈ 4.8 × 10⁻⁷ pc⁻³ y⁻¹ for a PBH with a fireball Lorentz-factor f = 10^4.3. This limit is applicable to PBHs in the halo of the Galaxy to distances ≲ 2 kpc, and dispersion measures ≲ 80 pc cm⁻³ . We also find a source-agnostic rate limit ≲ 0.25 events y⁻¹ deg⁻² for pulses of duration < 3 s, and having apparent energy densities ≳ 2.6 × 10⁻²³ J m⁻² Hz⁻¹ at 38 MHz. / Ph. D.

Primordial black holes (PBHs) are expected to form in the very early universe, and therefore, we could use them to probe pre­BBN times. Astrophysical observations permit the placement of bounds on the abundance of PBHs. Combining these bounds with predictions of PBH formation and the resulting PBH abundances, we explore the possibility to constrain an epoch of early matter domination (MD). For this analysis, we incorporate the recently obtained and most stringent bounds on PBHs from 21 cm observations. We assume density perturbations described by a near­power­law form of the curvature power spectrum consistent with the Planck 2018 results and derive the corresponding PBH mass function using recent results from the literature. To place constraints on an early matter­dominated (MD) epoch, we compare this with up­to­ date bounds on PBHs, adding those from 21 cm astronomy, using a formalism to apply monochromatic constraints to extended mass functions. We find that 21 cm bounds on PBHs may probe early MD if it lasts for a duration of at least ∼ 12 e­folds and ends at a reheating temperature T_min &lt; T_reh ≲ 105.5 GeV, where Tmin depends on the duration of the MD, the spectral index ns, and αs. For optimistic assumptions, our derived limits on the spectral index ns extend into the Planck 2018 67% upper confidence region of n_s = 0.9649 ± 0.0042, nearly reaching its median n_s = 0.9649, if we neglect the running αs. Our bounds on the running α_s show that early MD is incompatible with positive running α_s ≳ 0. / Primordiala svarta hål förutsäges ha skapats i det mycket tidiga universum. Ur detta förstår vi att dessa svarta hål ger oss en länk till universums tillstånd under deras skapelse och att vi genom studiet av primordiala svarta hål kan skymta detta tillstånd. Restriktioner på mängden primordiala svarta hål i universum kan beräknas utifrån astrofysiska observationer. Jämförelse av dessa restriktioner med teoretiska förutsägelser av vilka mängder av primordiala svarta hål som skapats vid olika tillstånd medger att vissa tillstånd utesluts. I detta arbete undersöker vi just detta samband och avser etablera huruvida de starkaste restriktionerna på primordiala svarta hål, speciellt från 21 cm observationer av universum, kan utesluta ett kosmologiskt scenario med tidig materiadominans. För att göra detta antar vi att störningar i den annars homogena energidensiteten kan beskrivas med hjälp av ett nästan­storleksoberoende potenssamband vars form ges av resultat från Planck kollaborationen 2018. Givet denna potenslag beräknar vi en form för de primordiala svarta hålens massdistribution medelst resultat från den vetenskapliga litteraturen. Vi jämför sedan dessa förutsägelser med de senaste mängdrestriktionerna på primordiala svarta hål, inkluderat dem från 21 cm observationer, för att erhålla nya begränsningar på tidig materiadominans. Vi tar i beaktande att massdistributionen av primordiala svarta hål inte är monokromatisk, utan utbredd. Resultatet av denna analys är att tidig materiadominans är inom räckhåll för att uteslutas medelst studier av primordiala svarta hål om perioden av materiadominans är längre än ∼ 12 e­folds och att den slutar vid återuppvärmningstemperaturen T_min &lt; T_reh ≲ 10^5.5 GeV, där Tmin beror på hur länge den tidiga materiadominansen varar, på det spektrala indexet ns och det spektrala indexets derivata αs. För optimistiska antaganden äventyrar dessa restriktioner på materiadominans resultat från Planck kollaborationen 2018 i det att det övre konfidensintervallet på det spektrala indexet n_s = 0.9649 ± 0.0042 är exkluderat, och att förändringen av detta index, α_s, inte kan vara positiv då vi låter storleksordningen minska.

Astrophysical phenomena such as self-annihilation of primordial black holes (PBHs), gamma ray bursts (GRBs), and supernovae are expected to produce single dispersed pulses detectable in the low end of the radio spectrum. Analysis of these pulses could provide valuable information about the sources, and the surrounding and intervening medium. The Eight-meter-wavelength Transient Array (ETA) is a radio telescope dedicated to the search for these pulses in an 18 MHz bandwidth centered at 38 MHz. ETA consists of 10 dual-polarized active dipoles providing an all-sky field of view. This thesis describes the results of a search campaign using ETA, namely, a Crab giant pulse (CGP) search. CGPs are already known to exist, and thus provide an excellent diagnostic for system performance. We found 11 CGP candidates in 14 hours of data. Although there has not been a single compelling detection (signal-to-noise ratio > 6), our analysis shows that at least a few of these candidates may be CGPs. We also explain the analysis preparation for PBH and GRB searches. Additionally, we describe the instrument and a software "toolchain" developed for the analysis of data that includes calibration, radio frequency interference (RFI) mitigation, and incoherent dedispersion. A dispersed pulse simulation code was developed and used to test the toolchain. Finally, improvements are suggested. / Master of Science