Periodic Reporting for period 1 - UniverScale (Sub-percent calibration of the extragalactic distance scale in the era of big surveys)
Reporting period: 2021-11-01 to 2023-04-30
Cepheids and eclipsing binaries are one of the best cosmic distance indicators. Eclipsing binaries offer measurements of absolute distances therefore they are the best targets to calibrate all other methods necessary to determine the Hubble constant. We will use these two geometrical techniques to measure geometrical distances to several objects and calibrate other methods including SN Ia and AGNs. This will allow us to significantly improve precision of the determination of the expansion of the Universe. In addition AGNs will be used to determine much larger distances than those calculated with SN Ia and as the result to determine the Hubble constant independently.
What is the problem/issue being addressed?
The aim of the AGN part of the project is to measure the Hubble constant using quasars (specifically, the continuum time delays in the broad band quasar spectra) . This method is potentially attractive since it does not require any cross-calibration with other methods, works directly from time-dependent quasar spectra. The current tension between the key measurements of Hubble constant is of order of 5 %, so our measurements to be successful needs to achieve better accuracy.
Why is it important for society?
Mankind wants to know the properties of the Universe. This is a natural curiosity even if it does not bring immediate benefits. Right now the observations show that the Universe consists in 5 % of the normal baryonic material which astronomers can directly observe, in about 30 % of dark matter which surrounds each galaxy as a bubble, and in 65 % of the dark energy. This dark energy is causing the accelerated expansion of the Universe. Initially astronomers postulated that this dark energy is well represented by the cosmological constant introduced by Albert Einstein. However, some of the most recent measurements seem to show a tension with this model, implying that the dark energy could evolve. Validation of this new claim will affect all our views on the future of the Universe evolution
What are the overall objectives?
As we aim to measure the Hubble constant which would test whether the dark energy evolves or not, we developed a unique strategy. In order to achieve this goal we need to use new observational data with the specifically planned strategy since they issue is not to achieve many measurements but to avoid potential systematic errors. In order to fully exploit these data we need to develop models since the observed quasar spectra are a combination of the accretion disk continuum and the cloud emission, and only after disentangling these two components we can successfully measure the Hubble constant.
What is the problem/issue being addressed?
The aim of the AGN part of the project is to measure the Hubble constant using quasars (specifically, the continuum time delays in the broad band quasar spectra) . This method is potentially attractive since it does not require any cross-calibration with other methods, works directly from time-dependent quasar spectra. The current tension between the key measurements of Hubble constant is of order of 5 %, so our measurements to be successful needs to achieve better accuracy.
Why is it important for society?
Mankind wants to know the properties of the Universe. This is a natural curiosity even if it does not bring immediate benefits. Right now the observations show that the Universe consists in 5 % of the normal baryonic material which astronomers can directly observe, in about 30 % of dark matter which surrounds each galaxy as a bubble, and in 65 % of the dark energy. This dark energy is causing the accelerated expansion of the Universe. Initially astronomers postulated that this dark energy is well represented by the cosmological constant introduced by Albert Einstein. However, some of the most recent measurements seem to show a tension with this model, implying that the dark energy could evolve. Validation of this new claim will affect all our views on the future of the Universe evolution
What are the overall objectives?
As we aim to measure the Hubble constant which would test whether the dark energy evolves or not, we developed a unique strategy. In order to achieve this goal we need to use new observational data with the specifically planned strategy since they issue is not to achieve many measurements but to avoid potential systematic errors. In order to fully exploit these data we need to develop models since the observed quasar spectra are a combination of the accretion disk continuum and the cloud emission, and only after disentangling these two components we can successfully measure the Hubble constant.
CAMK
The recruitment took slightly more time than anticipated, but we have been very successful in hiring very good scientists. All but two positions are filled. The remaining positions (senior and phd) will be announced soon (in a month).
The team achieved a very important step the calibration of the surface brightness – color relation based on eclipsing binaries for stars with a wide range of spectral types. This calibration will be extensively used in the following years to measure geometrical distances. We also calibrated P-L relations for several different types of pulsating stars in several passbands. We also dedicated a lot of efforts in writing applications for observing time and preparations for observations with our own telescopes at Cerro Armazones Observatory in Chile. These preparations include new software for planning and executing observations which will significantly improve both the quantity and quality of the data.
OBP PARIS
The research progress has been satisfactory at Paris Observatory over the first 18 months of the project. The recruitment of the PhD student Mrs Garance Bras (working on pulsating RR Lyrae stars and Cepheids) and the post-doctoral researcher Dr Anton Afanasyev (working on distance determination of SN Ia galaxies) went smoothly. Their recruitment however took slightly more time than initially planned, as they started their contracts in October and November 2022, respectively).
Over the reporting period, the collaboration within the UniverScale project with our Polish and Chilean colleagues has been vibrant, with many shared publications on various topics related to the distance scale.
UDEC
Our team at Universidad de Concepción, has worked on various classes of stellar distance indicators (Cepheids, RR Lyrae stars) in view of improving their usefulness for deriving sub-percent distances. Our particular focus was on the problem of how binarity among classical Cepheids, known to be as high as 80%, affects the use of Cepheid period-luminosity (PL) relations in optical and near-infrared photometric bands; this potential very serious problem has never been systematically tackled in the literature. We found that the effect of Cepheid binarity on distances measured from Cepheid PL relations in the near-infrared is less than 1%, a very important result for the aim of our project to achieve a sub-percent calibration of the distance scale (2 papers published)- We further worked on producing Cepheid PL relations in the Sloan filters, so far not available, which will become very important in the next years with the start of operations of the 8-m LSST telescope in Chile.- Work to improve the Baade-Wesselink method has also been a major part of our work, leading to interesting results and that we are approaching the project's goal to obtain distances of Cepheids accurate to 1-2% from this technique (1 paper published).
CFT & HITS
In this reporting period our action in the context of AGN application to cosmology was confined to the development of the suitable methodology. This is a crucial part as the method of using the quasar continuum time delay method to determine the Hubble constant was proposed in 1998 but was never implemented successfully. CFT in collaboration with HITS prepares the simulation software. Each of the institutes (CFT and HITS) does that independently but we slowly approach the moment of careful comparison of the simulations. CFT develops the simulation code which is based on Timmer-Koening algorithm of the generation of the incident lightcurve, we describe numerically the effect of the disk reprocessing without the use of the standard disk response function, and the created lightcurves are sampled with the adopted cadence. We also optionally include the Broad Line Region scattering. The time delay between the wavelength bands is determined with standard approach like ICCF or Chi2. The code developed by HITS is simpler in the aspect of using the analytic transfer functions, but it is more advanced in the time delay determination, and it contains the filter profiles. The simple comparison seems to suggest that our simulation imply larger errors for a similar setup. We are now starting the comparison more systematically, for the same global source parameters.
The recruitment took slightly more time than anticipated, but we have been very successful in hiring very good scientists. All but two positions are filled. The remaining positions (senior and phd) will be announced soon (in a month).
The team achieved a very important step the calibration of the surface brightness – color relation based on eclipsing binaries for stars with a wide range of spectral types. This calibration will be extensively used in the following years to measure geometrical distances. We also calibrated P-L relations for several different types of pulsating stars in several passbands. We also dedicated a lot of efforts in writing applications for observing time and preparations for observations with our own telescopes at Cerro Armazones Observatory in Chile. These preparations include new software for planning and executing observations which will significantly improve both the quantity and quality of the data.
OBP PARIS
The research progress has been satisfactory at Paris Observatory over the first 18 months of the project. The recruitment of the PhD student Mrs Garance Bras (working on pulsating RR Lyrae stars and Cepheids) and the post-doctoral researcher Dr Anton Afanasyev (working on distance determination of SN Ia galaxies) went smoothly. Their recruitment however took slightly more time than initially planned, as they started their contracts in October and November 2022, respectively).
Over the reporting period, the collaboration within the UniverScale project with our Polish and Chilean colleagues has been vibrant, with many shared publications on various topics related to the distance scale.
UDEC
Our team at Universidad de Concepción, has worked on various classes of stellar distance indicators (Cepheids, RR Lyrae stars) in view of improving their usefulness for deriving sub-percent distances. Our particular focus was on the problem of how binarity among classical Cepheids, known to be as high as 80%, affects the use of Cepheid period-luminosity (PL) relations in optical and near-infrared photometric bands; this potential very serious problem has never been systematically tackled in the literature. We found that the effect of Cepheid binarity on distances measured from Cepheid PL relations in the near-infrared is less than 1%, a very important result for the aim of our project to achieve a sub-percent calibration of the distance scale (2 papers published)- We further worked on producing Cepheid PL relations in the Sloan filters, so far not available, which will become very important in the next years with the start of operations of the 8-m LSST telescope in Chile.- Work to improve the Baade-Wesselink method has also been a major part of our work, leading to interesting results and that we are approaching the project's goal to obtain distances of Cepheids accurate to 1-2% from this technique (1 paper published).
CFT & HITS
In this reporting period our action in the context of AGN application to cosmology was confined to the development of the suitable methodology. This is a crucial part as the method of using the quasar continuum time delay method to determine the Hubble constant was proposed in 1998 but was never implemented successfully. CFT in collaboration with HITS prepares the simulation software. Each of the institutes (CFT and HITS) does that independently but we slowly approach the moment of careful comparison of the simulations. CFT develops the simulation code which is based on Timmer-Koening algorithm of the generation of the incident lightcurve, we describe numerically the effect of the disk reprocessing without the use of the standard disk response function, and the created lightcurves are sampled with the adopted cadence. We also optionally include the Broad Line Region scattering. The time delay between the wavelength bands is determined with standard approach like ICCF or Chi2. The code developed by HITS is simpler in the aspect of using the analytic transfer functions, but it is more advanced in the time delay determination, and it contains the filter profiles. The simple comparison seems to suggest that our simulation imply larger errors for a similar setup. We are now starting the comparison more systematically, for the same global source parameters.
Achieved:
1) Precision calibration of the SBCR based on eclipsing binaries (paper #1)
2) Absolute calibration of secondary distance indicators (papers #17 and 23)
3) The best so far estimation of the impact of binarity on the distance determination with Cepheids (paper #18)
Expected results
The expected results did not change and will be the same as described in our proposal.
1) Precision calibration of the SBCR based on eclipsing binaries (paper #1)
2) Absolute calibration of secondary distance indicators (papers #17 and 23)
3) The best so far estimation of the impact of binarity on the distance determination with Cepheids (paper #18)
Expected results
The expected results did not change and will be the same as described in our proposal.