The cosmic microwave background (CMB) is thermal radiation coming from an early stage of the Universe evolution. The small deviations in intensity and polarisation of the CMB from point to point on the sky carry unique information about the Universe at the moment of decoupling of radiation from baryonic matter around 380 000 years after the Big Bang. Driven by the mechanism of gravitational instability these tiny fluctuations in matter distribution formed the observed large-scale structure and galaxies. In the properties of CMB there are also encoded informations about interactions of CMB photons, travelling from the last scattering surface, with the large-scale structure. It gives us an opportunity to study the evolution of the structure.

In our department we are studying statistical properties of CMB maps, such as statistical isotropy and Gaussianity, as well as CMB anisotropy coming from interplay of CMB with the large-scale structure of the Universe. In particular, we study the CMB gravitational lensing effect which enables to trace distribution and evolution of dark matter and test validity of General Relativity. The effect is also a source of serious contamination for a search for the signatures of the primordial gravitational waves in CMB maps. For this reason there is a need of better understanding and reducing the lensing component from the maps. In these studies we use the state-of-the art data from the Planck experiment and galaxy catalogues from projects such as the Dark Energy Survey and Herschel Extragalactic Legacy Project. We are also involved in preparations for the analysis of data from forthcoming CMB experiments and galaxy surveys.

From cosmic microwave background radiation temperature fluctuations to filaments, voids, and superclusters of galaxies we try to map (and study) the distribution of mass in space. At this point astrophysicists are certain that galaxies are not uniformly distributed in space. The observed large scale structure evolves with time, and depends both on cosmological parameters and on the formation and evolution of galaxies. In our department we are using the two-point correlation function to trace the dependence of large scale structure on galaxy properties such as luminosity, color, and stellar mass. Using galaxy clustering we determine the average dark matter halo mass of a given galaxy population, connect observed galaxy populations at different epochs, and, in a longer term, constrain cosmological parameters and galaxy evolution models. Moreover, we are focused on the early epochs of galaxy formation - using large high-redshift surveys such as VIMOS Ultra Deep Survey (VUDS) and VIMOS Public Extragalactic Redshift Survey (VIPERS).

Understanding of the origin of accelerated expansion of the Universe (known as dark energy problem) is one of the most important challenges in modern cosmology and in fundamental physics. Lacking clear theoretical guidance in this respect we need to understand and describe the dynamics of the Universe in the most accurate and precise way using currently available observational data. At present, standard candles - Supernovae Ia and standard rulers (BAO, CMB acoustic peaks) serve this purpose. However, in order to break degeneracies in various cosmological parameters (curvature, mass density parameter, neutrino mass, Hubble constant) and trying to be free from prior assumptions on cosmological model (typically LCDM model) one needs new probes, which are alternative and complementary to the standard ones.

In the department we are developing the following new cosmological probes: strong gravitational lensing systems combined with stellar kinematics (complementary to time-delay method), mili-arcsecond compact radio-sources hosting intermediate luminosity quasars, quasars observed in UV and X-rays calibrated as standard candles, cosmic chronometers (passively evolving galaxies), gravitational wave signals from inspiralling compact binaries as standard candles and lensed gravitational wave signals as multimessengers. We are also using and developing non-parametric reconstruction techniques of expansion history of the Universe and consistency tests for the assumptions like LCDM model or validity of Friedman-Lemaitre-Robertson-Walker metric. Our interests extend to local measurements of cosmic curvature and testing validity of General Relativity on cosmological scales or tests of Lorentz invariance with extragalactic sources.

Our research at the NCBJ is focused on gravitational waves (GWs) which are disturbances in the fabric of spacetime as predicted by Einstein's general theory of relativity (GR). There are various sources of GWs but our group is working on radiation emitted from neutron stars. A neutron star is a collapsed core of a massive star which is supported by the neutron degenerate pressure. These objects emit GWs whenever there is a time-varying quadrupole moment. The GR predicts two polarization states of GWs and they are known as plus (+) polarization and cross (x) polarizations. But GR has some limitations and hence there are many attempts to study some alternative theories of gravity.

Currently, we are studying one of such alternative theories of gravity called Jordan Brans Dicke theory (JBD). This theory incorporates the Mach's principle and hence gravitational constant (G) is not a constant anymore. In general, a metric theory can have six polarization states but there are only three in the JBD theory, two tensor polarizations and one scalar polarization. Unlike in GR, the scalar polarization has also some contribution from the dipole moment. We are in the process of developing computer codes (taking JBD theory into account) to analyze data from LIGO Virgo O3 run.

Multi-messenger is a fairly new branch of observational astronomy. It started in the late 60s with joint observation in electromagnetic spectrum and neutrino detectors. But it is not limited to observations like that. On August 17, 2017, the LIGO-Virgo detector network observed a gravitational-wave signal from the inspiral of two low-mass compact objects consistent with a binary neutron star (BNS) merger: the first multi-messenger detection of colliding neutron stars.

Multi-messenger astronomy considers different astrophysical processes, and thus reveals different information about their sources. Previously it was electromagnetic radiation, neutrinos, and cosmic rays. Now the gravitational waves enriches the source of information about astronomical events. As far as the GW170817 is the only confirmed merging of neutron stars with electromagnetic counterpart, but it still is better then for merging of black holes - because there is no evidence of their electromagnetic counterparts.

The possibility of electromagnetic radiation as result of neutron stars or neutron stars and stellar mass black holes mergers was suggested by Li-Xin Li and Bohdan Paczyński in 1998. Additional to visual events this is an important source of nucleosynthesis of heavy nuclei.

Four sites (LHO, LLO, Virgo, KAGRA) together form a global network of ground-based GW detectors. The LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA Collaboration jointly analyze the data in real time to detect and localize transients from compact binary mergers and other sources.

Binary neutron star mergers spend several minutes in the spectral band of the Advanced ground-based gravitational-wave detectors. For some loud and nearby mergers, it is possible to detect them several tens of seconds before merger. This time is used to point the detectors of electromagnetic radiations (optical telescopes, gamma detectors) to possible sky positions of merging binary neutron stars.

What is most important with analysis of binary neutron star mergers is that, optical spectrum carries the information about how the process has ended, but gravitational waves carries information about motions of the mass within the source, so how the process has started. What we are hoping for, is to observe tidal deformation of gravitational wave signals due non-point mass distribution in neutron stars, which would allow us to impose better estimates on the neutron star equation of state.

To better understand the nature of neutron star mergers, at NCBJ we are working on the POLAR-2 space instrument, which is designed to observe polarization of short-GRBs, which as we know originate from the mergers. Polarization of short-GRBs is tightly linked to the magnetic field during the merger. Knowledge about the magnetic field within the merger would give us a better understanding of the most violent phase of the mergers. POLAR-2, an ESO instrument, is scheduled to be operational in the second half of 2020s. NCBJ is a member of the POLAR-2 collaboration and was also involved in the POLAR instrument.

Since it is generally assumed that detectable electromagnetic (or neutrino) emission starts shortly after merger, a pre-merger gravitational-wave detection would provide early warning of an coming electromagnetic transient and might make it possible for automated follow-up facilities to capture any prompt emission from the merger environment, the jet, and other unknown activity. Alerts are distributed through NASA’s General Coordinates Network (GCN) and Scalable Cyberinfrastructure to support Multi-Messenger Astrophysics (SCiMMA). Virgo-Polgraw group members (prof. Andrzej Królak, dr Adam Zadrożny ) were involved in piloting of electromagnetic follow-up called Looc-Up (2009-2014) and in preparation of infrastructure for sending alerts to astronomers.

Virgo-Polgraw is a Polish team of researchers analyzing the data collected by the LIGO and Virgo detectors in search for gravitational waves predicted by the general theory of relativity by Albert Einstein.

But also scientists of Virgo-POLGRAW as part of the LVK-collaboration take duties of service in time of detectors observation to swiftly react to events. If in time of a duty there is a signal about compact stars merging, scientists take the procedure of verification of the event and after verification send the public alert. This alert should initiate a procedure of observations by worldwide community members.

Studying galaxy formation and evolution through cosmic time is a cornerstone of modern cosmology and astrophysics. Moreover, it is one of the most active interdisciplinary fields of science, covering the physical and chemical complexities that a galaxy undergoes during its lifetime, from formation to death.Galaxies emit radiation across the electromagnetic spectrum, from \gamma rays to radio waves. In BP4, we make use of the extensive multi-wavelength catalogues from the Herschel Extragalactic Legacy Project (HELP), the Low-Frequency Array (LOFAR) and AKARI, covering hundreds of millions of galaxies observed from the UV to radio out to redshift 5, to derive physical properties of the galaxies that they contain (dust attenuation, star-formation rates, stellar masses, etc). We couple these photometric catalogues with spectroscopic data from the Atacama Large (sub)Millimetre Array (ALMA) and VIMOS Public Extragalactic Redshift Survey (VIPERS), which allow us to directly examine galaxies’ interstellar medium (ISM), including metallicities and gas properties. From these data, we can better constrain the physical relations that govern them and the implications these relations have on their evolution. The wealth of data also allows us to examine the most extreme galaxies, such as Ultra-luminous infrared galaxies (ULIRGs). The analysis we perform with these catalogues will also prepare us for the future surveys from the Vera C. Rubin Observatory.Our work employs state of the art techniques. Using the latest spectral energy distribution (SED) fitting codes, we test the non-universality of dust attenuation laws to derive a more general dust extinction behaviour to better describe the largest possible galaxy sample. Modern image analysis techniques are used to derive the morphological and structural parameters, allowing an exploration into how the form of a galaxy influences its internal processes and how its environment influences its morphology. Cutting edge machine learning techniques are also used to allow us to generate new insights and rapidly process the unprecedented volumes of data that we have now and will have in the future.

As part of research on active galactic nuclei (AGN) and quasars (QSO), we develop automatic selection and redshift estimation methods of AGN and QSO in photometric surveys. Our main research interests include:

• reliable and interpretable machine learning models
• photo-z uncertainty
• deep learning in low-dimensional spectroscopy and spectroscopic feature reconstruction from photometric data
• model testing techniques
• visualisation of high-dimensional feature spaces
• extrapolation with predictions from spectroscopic surveys to much deeper photometric ones
• type II quasar and AGN selection
• outliers and unknown objects detection
• problems with small training samples
• SED fitting

Almost 50 years after confirmation of the existence of the neutron star, the equation of state of the matter that comprises these stars is still under discussion. In neutron stars, the density in the center of the star exceeds a few times the nuclear density. Many theoretical models of the equation of state (EOS) of superdense matter have been proposed. Astronomical observations are the only way to verify the EOS of neutron stars, because in Earth laboratories we are unable to reproduce conditions similar to neutron star interiors. A very important property of theoretical models is the existence of a maximum mass for the neutron star and a unique mass-radius relation for each assumed EOS. There exist various methods to constrain the EOS with astronomical observations.

There exist methods that allow for simultaneous mass and radius determination, and consequently, to determine the EOS. One such method is fitting observed spectra with model atmospheres.

Our group develop an unique model atmosphere of hot neutron star and accretion disks (ATM24 code). This code calculates the radiative transfer equation in a plane-parallel geometry. It takes into account the effect of Compton scattering on free, relativistic electrons, where initial photon energies can approach the electron rest mass. We assume the equation of state of ideal gas being in local thermodynamical equilibrium (LTE). We solve the model atmosphere together with hydrostatic and radiative equilibrium equations. The influences of the magnetic field and accretion onto the neutron star are not included. Our code takes into account energy-dependent opacities from hydrogen, helium and heavy element ions in LTE. The ionization equilibrium is fully solved. We neglect the effect of electron degeneracy, which is unimportant in the hot atmospheres.

Our theoretical spectra could be fitted to the observed spectra of hot neutron stars in different type of objects like X-ray bursters, isolated neutron stars or X-ray transient in the period when the neutron star is not accreting matter.

Stars are born in giant molecular clouds of dust and gas where the temperature and density are ideal for their formation. Because they are deeply embedded in the cloud, we have to rely on the infrared and millimeter part of the spectrum to have insights on the physical processes occurring during the early-stages of star formation.

High-mass stars emits UV photons which ionize the surrounding medium and lead to an HII region. This HII region further expands and sweeps-up the surrounding material to finally form a circular structure often called HII bubble, despite the fact that the geometry of such structure remains uncertain. When the layer of material enclosing the HII region reaches a certain density, it fragments to form the next generation of stars.

Despite the fact that 30% of Galactic high-mass sources are found at the edges of these HII regions, we are still not sure about the true effect of the pressure of the HII regions which is exterted on the layer of material, and therefore, on the star-formation occurring in it.

At NCBJ, we use the data provided by Herschel from The Herschel Imaging Survey of OB Stars (HOBYS) and the Herschel survey of the Galactic Plane (Hi-GAL) to understand the properties of HII regions from large to small scales. This is important as it give us crucial information on the mass fo the cores, the ionisation pressure of the HII region and the properties of the filaments inside the PhotoDissociation Region.

To understand the fragmentation process down to 0.01pc and below, the current Galactic plane surveys do not have enough spatial resolution. Therefore, we use high-resolution interferometric observations of the population of cores around HII regions. Such observations are performed with interferometers such as the Atacama Large Millimetre Array (ALMA). Thanks to them, we can observe and understand how the cores are fragmenting and what are the drivers of this fragmentation (gravity, turbulence, magnetic field, …)