Primordial Black Holes May Hold Clues to the Universe’s Missing Mass
Primordial black holes formed after the Big Bang may account for dark matter, and researchers are studying their particle emissions to test this theory.
By Anubha Jain
Published : August 12, 2025 at 4:20 PM IST
Bengaluru: Even 50 years ago, scientists believed that everything in the universe was made of matter we could see—photons, neutrons, electrons, and protons. Today, with precise astrophysical and cosmological measurements, scientists can measure the energy density of the universe, which includes both visible and invisible components. These measurements reveal that visible matter accounts for only 5 per cent of the universe’s total energy density. The rest is invisible: about 25 per cent is dark matter (a form of matter that does not emit, absorb, or reflect light) and 70 per cent is dark energy (a mysterious force causing the accelerated expansion of the universe), which together make up 95 per cent of the universe's total energy content.
Against this backdrop, Ranjan Laha, Assistant Professor at the Centre for High Energy Physics, Indian Institute of Science, Bengaluru, spoke in detail with ETV Bharat about dark matter and its possible origins, focusing particularly on Primordial Black Holes (PBHs). A popular candidate that may constitute dark matter, or some part of dark matter, is PBH—hypothetical black holes formed in the very early universe, not from dying stars but from extreme density fluctuations. This possibility is currently being explored by researchers all over the globe, including Ranjan’s group.
Laha said that dark matter, which does not interact with light, explains why galaxies like the Milky Way rotate so fast without flying apart. Galaxies rotate at speeds that would tear them apart if only visible matter were present; dark matter provides the extra gravitational pull needed for stability. Its existence is certain, but its true nature remains unknown. Scientists worldwide are exploring theoretical possibilities, one of which is PBHs.
Formation of primordial black holes (PBHs)
These primordial black holes are hypothesised to have formed in the early universe through as-yet-unknown physics. According to the professor, PBHs could serve as dark matter, as they exhibit many of the key properties expected of it.
Larger masses have proportionally larger radii. However, if enough matter is compressed into a small enough space, it collapses into a black hole, Laha said, referencing Einstein’s general relativity. For example, if the Sun’s radius were reduced to just 3 km, it would become a black hole. PBHs seem to follow the same theory. Stellar black holes, formed from dying stars, can’t be lighter than about two solar masses, but PBHs could have a far wider mass range—including extremely small ones.
To give context, the Sun’s mass is about 10^{33} grams, while the PBHs studied by Laha’s group have masses around 10^{16} to 10^{17} grams—that’s a trillion trillion times lighter than the Sun. Stephen Hawking proposed that black holes have a temperature and can emit particles—a phenomenon called Hawking radiation. Though not yet experimentally proven, it is widely accepted as theoretically sound.
By studying the particle signatures from PBH evaporation, Ranjan Laha's team aims to probe dark matter in the form of PBHs. Dark matter constitutes about 25 per cent of the universe’s energy density, and PBHs could theoretically account for a similar share. This can be tested by constraining the possible masses of PBHs that might contribute to dark matter. When PBHs evaporate, they inject energy into the universe in the form of particles, which must appear somewhere in cosmic observations. Detecting these signals could reveal what fraction of dark matter is composed of PBHs.
Promising mass ranges and observational signature
When asked about the most promising mass range for PBHs to constitute a significant fraction of dark matter, Laha explained that recent data indicate certain mass regions—from about four times around 10^{17 } g to 10^{21} g—where PBHs could account for 100 per cent of dark matter. In other mass ranges, PBHs might still make up a substantial fraction, such as 10 per cent or even more than 1 per cent of dark matter. Typically, black holes are formed by the death of massive stars; the entire mass of PBHs implodes into an extremely dense, compact region.
Early universe conditions and PBH formation
Discussing the conditions in the early universe that could have favoured the formation of PBHs, Laha explained that if small regions of the early universe had extremely high density, they could have collapsed under their own gravity to form black holes. Density fluctuations refer to variations in matter concentration in different regions of the early universe.
While the large-scale structure of the universe—on scales of hundreds of megaparsecs (1 parsec ≈ 3.26 light-years and 1 megaparsec is equal to 1 million parsec)—is isotropic and homogeneous, smaller scales in the early universe might have contained significant density fluctuations. If regions at small scales had high enough density in those early times, black holes could have formed. "However, we currently lack direct evidence of such high-density regions at these scales," said Laha. Scientists, therefore, simulate scenarios assuming such conditions and then search for possible observational signatures of PBHs through various astrophysical and cosmological probes.
Inflation and small-scale fluctuation
Discussing specific inflationary models and early-universe fluctuations that could lead to the formation of PBHs, Laha noted that while inflation is being tested at large cosmological scales, its behaviour at much smaller scales—down to kilometres or even atomic dimensions—remains uncertain. Inflation refers to the rapid expansion of the universe in its earliest moments, which smoothed out large-scale structures but may have left small-scale irregularities.
Certain theoretical scenarios predict that such small scales could have experienced regions of very high density in the early universe, potentially leading to PBH formation. Detecting PBHs would be one way to test these ideas.
Observational advances: InPTA and gravitational waves
Highlighting observational advances, Laha pointed to the Indian Pulsar Timing Array (InPTA) collaboration, where India plays a central role. InPTA uses the upgraded Giant Metrewave Radio Telescope to monitor millisecond pulsars (MSPs) and search for ultra-low-frequency (nanohertz) gravitational waves. Pulsars are rapidly rotating neutron stars that emit beams of radio waves; timing their pulses can reveal gravitational wave signals. Such measurements could reveal collisions of supermassive black holes in distant galaxies. People are trying to connect such observations to PBHs, too.
As for whether any detected signal will be a confirmed signature of such events, Laha said it is still uncertain. However, with rapid progress in the past two years, scientists are actively working toward answering that question.
PBHs and the intergalactic medium
The professor explained that his team is investigating whether PBHs exist and how their presence could affect the intergalactic medium—the matter that lies between galaxies. In particular, they study how this medium’s temperature might change over time.
Observations already provide measurements of this temperature, and his work involves analysing these data and developing theoretical models to interpret them. He is also focusing on light primordial black holes with masses around 10^{16} g. For these, the team looks for signatures of Hawking evaporation to assess their potential effects on the universe.
Cosmic implications of PBH discovery
When asked how discovering that PBHs constitute a portion of dark matter might influence our understanding of galaxy evolution and cosmic history, Prof. Laha said that such a finding would be transformative. At present, humans understand only about 5% of the universe’s composition. Identifying the nature of just 25% of dark matter can also fundamentally change our view of the cosmos. PBHs can serve as a probe of the universe’s properties at specific scales. Statistically, detecting PBHs would provide insights into how the universe’s density was distributed in small regions—on the order of a few kilometres—shortly after the Big Bang.
Referring to big bang nucleosynthesis—the process by which light elements like hydrogen and helium were formed in the early universe and which occurred roughly three minutes after the Big Bang—Professor Laha noted that PBHs would have formed much earlier. They are believed to have originated less than a second after the Big Bang, during a time when space was not perfectly homogeneous.
Stability and Hawking radiation
Laha’s group is exploring the possibility that these PBHs—ancient black holes formed in the universe’s first moments—could be the elusive dark matter. Although PBHs could only have formed right after the Big Bang, they are thought to be remarkably stable. Those with masses greater than about 5 times 10^{12} kg may have survived to the present day, while lighter ones are expected to have evaporated through Hawking radiation, the theoretical thermal emission caused by quantum effects near a black hole’s event horizon. Hawking radiation is a unique phenomenon that combines gravity and quantum mechanics.
Professor Laha said, “While there are mass ranges of PBHs for which Hawking radiation is negligible, in the mass range I study, it is the key process under investigation. Black holes are often thought to only absorb matter, but Hawking proposed that they can also emit radiation. Our work aims to provide evidence that Hawking was right,” he added.
