A Telescope with no moving parts
The Low-Frequency Array (LOFAR) Core is concentrated in a field near Exloo, but its full extent is continental. It is a "software telescope," using more than 70,000 simple, stationary dipole antennas grouped into stations across Europe to observe the universe. The signals are digitized and sent via high-speed fiber optic networks to a central processor in Groningen. There, a supercomputer combines the data to simulate a single radio antenna that can be up to 2,000 kilometers in diameter. This technique, called interferometry, gives LOFAR extraordinary resolution without any moving parts.
The core near Exloo contains 24 of these stations in a dense configuration. Each station has two types of antennas. The Low Band Antennas (LBA) are simple wire dipoles that observe frequencies from 10 to 90 MHz. The High Band Antennas (HBA) are tiles that cover the 110 to 240 MHz range. This design intentionally avoids the 90-110 MHz gap, which is saturated by commercial FM radio broadcasts. Because the antennas are omnidirectional, software can "point" the telescope by selecting and processing signals from specific directions, allowing for multiple simultaneous observations.
Listening to the first light
One of LOFAR's main scientific objectives is to detect the faint radio whispers from the Epoch of Reionization (EoR). This period, in the first billion years after the Big Bang, is when the first stars and galaxies formed, filling the universe with light and ionizing the neutral hydrogen gas that filled space. LOFAR hunts for the redshifted 21-cm line emission from this primordial hydrogen. This signal is extremely weak, buried under astrophysical foreground signals that are several orders of magnitude stronger, requiring immense computational power to isolate.
Beyond the cosmic dawn, LOFAR's main science projects include surveying the low-frequency sky, studying cosmic magnetism, and detecting transient radio events like pulsars and fast radio bursts. It also observes the Sun to understand space weather and investigates the origins of ultra-high-energy cosmic rays by detecting the radio flashes they create in Earth's atmosphere. A major survey, the LOFAR Two-metre Sky Survey (LoTSS), is mapping the entire northern sky, aiming to list over 10 million radio sources. In 2021, a published map from this survey showed more than 25,000 active supermassive black holes in distant galaxies.