Narayan Khadka

PhD candidate, Cosmology, Astrophysics

Observational data indicate that the cosmological expansion is currently accelerating. They also indicate that in the recent past the expansion was decelerated. The standard spatially-flat ΛCDM model (Peebles 1984) is the simplest model consistent with these observations. In this model, dark energy — in the form of a cosmological constant (Λ) — dominates the current cosmological energy budget and powers the currently-accelerating cosmological expansion. In this model, above a redshift z of about 3/4, non-relativistic cold dark matter (CDM) and baryonic matter contributes more than Λ does to the energy budget and powered the then-decelerating cosmological expansion. While the observations are consistent with dark energy being time- and space-independent, they do not rule out slowly- evolving and weakly spatially-inhomogeneous dynamical dark energy, nor do they rule out mildly curved spatial hypersurfaces.



Significant constraints on cosmological parameters come from cosmic microwave background (CMB) anisotropy data — that primarily probe the z ∼ 1100 part of redshift space — as well as from baryon acoustic oscillation (BAO) observations — the highest of which reach to z ∼ 2.3 — and other lower-redshift Type Ia supernova (SNIa) and Hubble parameter [H(z)] measurements. Observational data in the intermediate redshift range, between z ∼ 2.3 and ∼ 1100, are not as constraining as the lower and higher redshift data, but hold significant promise. In the intermediate redshift range 2.3 < z < 1100, cosmological models are poorly tested. In this range there are handful of data sets. These include HIIG starburst galaxy data that reach to z ∼ 2.4, quasar angular size measurements that reach to z ∼ 2.7, gamma-ray burst (GRBs) observations that reach to z ∼ 8.2, and Quasar (QSO) X-ray and UV flux measurements that reach to z ∼ 7.5. We focus our research on GRB and QSO observations to test general relativistic cosmological dark energy models and try to see if GRB and QSO measurements can be reliable cosmological probes.

QSO research

Since 2017 we (me and Prof. Ratra, Group Webpage) are studying and using quasar's X-ray and UV flux measurements to constrain six different general relativistic dark energy cosmological models. These models include standard spatially-flat ΛCDM model to more complex scalar filed model (φCDM). QSO-flux measurements span redshift range of 0.009 < z < 7.5. These are one of the handful observations that can be used in cosmology and cover such a high redshift space. These data favor relatively high value of current matter density parameter (Ωm0) of the universe which leads to the tension between the hubble diagram of these quasars and the standard spatially-flat ΛCDM model with Ωm0 = 0.3. This is a very interesting cosmological result but it is too early to conclude that this tension is related to the inadequacy of the standard spatially-flat ΛCDM model. Our recent work shows that there is an issue standarding these data using L X - L UV relation, an observed correlation between X-ray and UV luminosity of a quasar which is a basis of our method. Free parameters associated with L X - L UV relation show cosmological model dependency which causes difficulty to standardize quasar. Currently, we are trying to resolve this issue. If we succeed to resolve this issue, these quasars can be a very useful high redshift cosmological probe.

GRB research

GRBs have been observed to high redshift, at least to z = 8.2. If it is possible to standardize GRBs, they can then be used as a cosmological probe to study a part of the universe which is not presently accessible to any other cosmological probe. For some GRBs the observed peak photon energy (Ep) and isotropic radiated energy (Eiso) are related through the Amati relation (Ep - Eiso relation). This correlation enable us to use GRBs to constrain cosmological model parameters. Currently, we have 220 GRBs which have spectral features that are required by the Amati relation. These 220 GRBs also favor significantly higher value of (Ωm0) which is inconsistent with most of the well-established cosmological probes such as cosmic microwave background (CMB) anisotropy, BAO, H(z), and supernova (SNIa) data. Our study shows that some of the GRBs (102 GRBs, see Khadka et al (2021)) prefer significantly high intrinsic disperson (≅ 0.5) to the Amati relation. So, these GRBs are not suitable for the cosmological purposes. Remaining 118 GRBs shows relatively low intrinsic dispersion and are consistent with the standard spatially-flat ΛCDM model. Constraints on cosmological parameters obtained using these 118 GRBs are significantly weeker than those well-established cosmological probes and we hope that future GRBs observations will provide tighter constraints.

BAO and H(z) constraints

We derive constraints on cosmological parameters from the BAO and H(z) data. We update BAO data when the new data are publicly available. For our recent results obtained using BAO and H(z) measurements, please see Khadka & Ratra (2021). We use these results for comparing to the GRB and QSO results we derive, to examine consistency between the GRB and QSO data constraints and constraints from the better-established BAO + H(z) cosmological probes.




In general, I focus on testing general relativistic cosmological dark energy models using observational data. Currently ,I use baryon acoustic oscillations (BAO) observations, Hubble parameters H(z) measurements, high redshift quasar's X-ray and UV flux measurements, and high redshift gamma ray burst observations.


B.S. Physics, Trichandra College, Nepal, 2012

M.S. Physics, Tribhuvan University, Nepal, 2016

PhD. Physics, Kansas State University, USA, (In progress)


  • Paintings: I am interested in painting. I paint natural landscapes and abstract art. I starded painting since my childhood. You can see some of my paintings in my gallery.
  • I love to play football.
  • I like to observe the nature.



Khadka, N., Martinez-Aldama M. L., Zajaček M., Czerny B., Ratra, B., Do reverberation-measured H𝛽 quasars provide a useful test of cosmology?,e-Print: 2112.00052

Cao S., Khadka, N., Ratra, B., Standardizing Dainotti-correlated gamma-ray bursts, and using them with standardized Amati-correlated gamma-ray bursts to constrain cosmological model parameters, e-Print: 2110.14840

Khadka, N., Ratra, B., Do quasar X-ray and UV flux measurements provide a useful test of cosmological models?, e-Print: 2107.07600

Khadka, N., Yu Z., Zajaček M., Martinez-Aldama M. L., Czerny B., Ratra, B., Standardizing reverberation-measured Mg II time-lag quasars, by using the radius-luminosity relation, and constraining cosmological model parameters, Mon.Not.Roy.Astron.Soc. 508 (2021) 4, 4722–4737

Khadka, N., Luongo, O., Muccino, M., Ratra, B., Do gamma-ray burst measurements provide a useful test of cosmological models?, J. cosmology Astropart. Phys., 2021, 042

Khadka, N. & Ratra, B., Determining the range of validity of quasar X-ray and UV flux measurements for constraining cosmological model parameters, Mon.Not.Roy.Astron.Soc. 502 (2021) 4, 6140-6156

Cao S., Ryan J., Khadka, N., Ratra, B., Cosmological constraints from higher redshift gamma-ray burst, H ii starburst galaxy, and quasar (and other) data, Mon.Not.Roy.Astron.Soc. 501 (2021) 1, 1520-1538

Khadka, N. & Ratra, B., Constraints on cosmological parameters from gamma-ray burst peak photon energy and bolometric fluence measurements and other data, Mon.Not.Roy.Astron.Soc. 499 (2020) 1, 391-403

Khadka, N. & Ratra, B., Using quasar X-ray and UV flux measurements to constrain cosmological model parameters, Mon.Not.Roy.Astron.Soc. 497 (2020) 1, 263-278

Khadka, N. & Ratra, B., Quasar X-ray and UV flux, baryon acoustic oscillation, and Hubble parameter measurement constraints on cosmological model parameters, Mon.Not.Roy.Astron.Soc. 492 (2020) 3, 4456-4468

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