Cosmography with the Einstein Telescope
Comparing achievable cosmological parameter estimates for ET-B and ET-C
by Marco Tompitak
In the last decades, there has been a large effort towards realizing the direct detections of gravitational waves. Once this field of physics has been opened up, as is expected to be done by the second generation detectors to be completed in the next few years, a completely new way of looking at the universe will become available. Though the second generation detectors will already be able to perform ground-breaking science, an even better detector will be desirable. Such a third generation detector, the Einstein Telescope (ET), is already undergoing a design study. This work focuses on the research that can be done with ET in the field of cosmology and contributes to the design study by analysing computer simulations of detections that will be performed by this detector. The main goal of the research described here is to make a comparison between the two currently available design concepts for ET, ET-B (essentially an upscaling of the designs of currently operational detectors) and ET-C (a so-called xylophone design, consisting of two detectors sensitive to different frequencies, combined to obtain a broadband detector), as regards their ability to estimate the cosmological parameters.
It is first shown that gravitational wave observations made by ET can be used to estimate the cosmological parameters with an accuracy of the same order of magnitude as electromagnetic observations have provided. Then a comparison is made between the accuracies provided by ET-B and ET-C. It is found that the extra width in the sensitivity band gained by employing ET-C rather than ET-B is completely lost unless a particular type of low-frequency noise, the gravity gradient noise, can be significantly reduced. This noise type imposes a lower cut-off frequency to the sensitivity of the detector of 10 Hz, below which confidence in the information coming from the detector is too low. If this cut-off stays in effect, ET-B proves the better design, estimating the cosmological parameters about 15% more accurately, on average. If this cut-off can, however, be reduced by a few Hz (the turning point is at around 7.7 Hz), ET-C will become the better option. With a cut-off frequency of 5 Hz, ET-C estimates the parameters with about 14% more accuracy and with 1 Hz, this number becomes 15%. The choice between ET-B and ET-C, at least as far as cosmology is concerned, thus depends on the ability to push down the cut-off frequency below 10 Hz.
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