10 years of LOFAR highlights: Improved upper limits on the 21 cm signal power spectrum of neutral hydrogen at z ≈ 9.1 from LOFAR

By André Offringa

13.8 billion years ago, our Universe was created in an event called the Big Bang. "Only" 0.5 billion years later, the Universe entered a pivotal stage. At that time, the Universe was filled with cold hydrogen, and the first objects, such as early stars and galaxies, had just formed. These objects started to heat the hydrogen, causing "bubbles" of heated hydrogen to form around the radiating sources. This process is called the Epoch of Reionization.

Published by the editorial team, 8 June 2020

A group of researchers is using LOFAR to improve our understanding of this early phase of our Universe. The bubbles of heated hydrogen leave a particular imprint behind in the signals that LOFAR receives, and can therefore be detected by LOFAR. However, this requires advanced modelling techniques and combining many observations to detect the weak imprint behind all the nearby bright objects.

In this highlight, researchers have shown that they have made a significant step forward. By analyzing 141 hours of observing, they have have been able to rule out, for the first time, that the imprint exceeds a certain brightness. They will continue using LOFAR to try and find direct proof of this mysterious era in the evolution of our Universe.

Schematic representation of the evolution of our universe, credit: NAOJ

The article this highlight is based on: Improved upper limits on the 21-cm signal power spectrum of neutral hydrogen at z≈9.1 from LOFAR F. G. Mertens, M. Mevius, L.V.E Koopmans, A. R. Offringa and others.
DOI: 10.1093/mnras/staa327

On 12 June 2020, LOFAR celebrates its tenth anniversary. The radio telescope is the world’s largest low frequency instrument and is one of the pathfinders of the Square Kilometre Array (SKA), which is currently being developed. Throughout its ten years of operation, LOFAR has made some amazing discoveries. It has been a key part of groundbreaking research, both in astronomy and engineering. Here we feature some – but definitely not all – of these past highlights, with surely more to come in the future.


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On June 13-17, the LOFAR Family Meeting took place in Cologne. After two years LOFAR researchers could finally meet in person again. The meeting brings together LOFAR users and researchers to share new scientific results.

Our renewed ‘Melkwegpad’ (Milky Way Path) is finished! The new signs have texts in Dutch on the one side and in English on the other side. The signs concerning planets have a small, 3D printed model of that planet in their centre.
#Melkwegpad @RTVDrenthe

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The background drawing shows how the subband correlator calculates the array correlation matrix. In the upper left the 4 UniBoard2s we used. The two ACM plots in the picture show that the phase differences of the visibilities vary from 0 to 360 degrees.

Daily image of the week: Testing with the Dwingeloo Test Station (DTS)
One of the key specifications of LOFAR2.0 is measuring using the low- and the highband antenna at the same time. For this measurement we used 9 lowband antenna and 3 HBA tiles.