This article was published in Scientific American’s former blog network and reflects the views of the author, not necessarily those of Scientific American
Neutrinos and gravitational waves could hold clues from a past era, but can we decipher them?
In "Too Hard For Science?" I interview scientists about ideas they would love to explore that they don't think could be investigated. For instance, they might involve machines beyond the realm of possibility, such as devices as big as galaxies, or they might be completely unethical, such as experimenting on children like lab rats. This feature aims to look at the impossible dreams, the seemingly intractable problems in science. However, the question mark at the end of "Too Hard For Science?" suggests that nothing might be impossible.
The scientist: Martin Bojowald, associate professor of physics at Pennsylvania State University and author of "Once Before Time: A Whole Story of the Universe."
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The idea: The Big Bang is often thought of by scientists as the beginning of everything, including time, making any questions about what happened beforehand nonsensical. Now exotic theories suggesting the existence of an era before the Big Bang are growing in number. If true, it might be possible to detect traces of this distant time.
The quest for signals from this bygone era faces many challenges. "Trying to make observations of anything that happened before the Big Bang is like looking through a wall of fire," Bojowald says. "The universe during the Big Bang was so dense and hot that signals of light and any other form of electromagnetic waves are scattered and deflected too much to transmit information."
The key would be focusing on signals from this past time that interact with matter only very weakly — "neutrinos and gravitational waves," Bojowald says. Neutrinos have already been detected directly, while "gravitational waves are expected to be detected within the next few years once Advanced LIGO [the Laser Interferometer Gravitational-Wave Observatory] is ready, and there are already concrete plans for more sensitive satellite-based detectors such as LISA [the Laser Interferometer Space Antenna]."
The problem: The problem with relying on signals that interact with matter very weakly is that they are extremely difficult to detect. For instance, when it comes to neutrinos, when even the largest current neutrino detectors are used, "we see only a small fraction of these particles," Bojowald says. To scan for any evidence of an era before the Big Bang, scientists would need to detect a significant fraction of neutrinos of all energies and their precise directions across the sky. So far, neutrino experiments can only determine either the precise directions of high-energy neutrinos or the approximate direction of a limited range of energies of neutrinos, and detecting old, very low energy neutrinos will likely involve extraordinarily large instruments.
When it comes to detecting gravitational waves, a space-based mission such as LISA would have a much easier time than a ground-based experiment such as LIGO. Unfortunately, budgetary woes apparently mean that NASA will not contribute its share of funding to LISA, meaning the European Space Agency will likely pursue a scaled-down mission.
The solution? "We will have to wait several decades until we have the required detectors available, but those detectors will be multi-purpose machines of interest for many questions of particle physics and cosmology, so they will eventually be available," Bojowald suggests.
The main problem when it comes to using such machines is understanding what hints they might uncover about the early universe. "The required theories, just like the detectors, are still being built," Bojowald says.
Instead of looking directly for gravitational waves, scientists could also try detecting their indirect effects instead — for instance, by looking at ripples they cause in the cosmic microwave background radiation. However, the cosmic microwave background we see only formed several hundred thousand years after the Big Bang. As such, any signals from beforehand might have grown weak before they had a chance to influence this radiation, so even more theoretical extrapolation might be required to discern any such clues.
Even if this bygone time existed, it may be impossible to get a good picture of what it once was like. A phenomenon that Bojowald calls "cosmic forgetfulness, based on a combination of quantum theory and space-time physics," could lead the universe to lose some of its past properties after the Big Bang and gain new ones independent of what it had before. In other words, there may be details of this past era that could never make it to the present.
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If you have a scientist you would like to recommend I question, or you are a scientist with an idea you think might be too hard for science, email me at toohardforscience@gmail.com.