Most of what humankind and other mammalian species on Earth experience of the Universe is primarily restricted to the part of the electromagnetic spectrum which our optical organs can register. Despite these limitations, we have found ways over the centuries which enable us to perceive the rest of the EM spectrum, to see both what is incredibly far away, and what is incredibly small, to constantly get a little bit closer to understanding what makes the Universe into what we can observe today, and what it may look like in the future.
An essential element of this effort are space telescopes, which gaze into the depths of the Universe with no limitations imposed by the Earth’s atmosphere, or human activity. Among the many uses of space telescopes, the investigation of the expansion of the Universe is perhaps the most fascinating, as this brings us ever closer to the answers to the most fundamental questions about not only its shape, but also to its future, which may include hitherto unknown types of matter and energy.
With the recently launched Euclid space telescope, another chapter is being opened in the saga on dark energy and matter, and their nature and effects on the Universe, as well as whether they exist at all. Yet how exactly do you use a space telescope to ferret out the potential effects of dark energy?
The Dark Universe
When Albert Einstein was working out his General Relativity theory, he had assumed that the Universe would be static, and introduced a cosmological constant to the field equations that would would balance out the expansion of the Universe in the model. When a number of years later Edwin Hubble demonstrated that the Universe is in fact expanding, which led to Einstein calling the cosmological constant his ‘greatest blunder’. Yet unbeknownst to Einstein, this cosmological constant would later be revived, as the factor that drives the expansion of the universe.
In both the case of dark matter and dark energy, their presumed existence is the result of the theoretical models that we have developed over the past decades. These models need dark matter and dark energy to accord with what we see. Since the 1990s increasingly refined observations of the Universe has allowed us to validate elements of these theories, with “dark matter” and “dark energy” essentially acting as placeholders until we can either demonstrate their existence, or develop a new model that does not require one or both of these.
Within the currently most prominent Lambda-Cold Dark Matter (ΛCDM) model, the presence of dark matter serves to explain observations made of galaxies, as their shape and rotation cannot be explained using just the observable luminous (baryonic) matter. Following Kepler’s Second Law, there should ultimately be an estimated 85% of dark matter in the Universe, with the rest being ordinary matter. This presumed existence of dark matter (the Cold Dark Matter in ΛCDM) is backed up by a number of other observations including of the cosmic microwave background (CMB) and gravitational lensing, even if we do not know what this matter is.
Dark matter is postulated to not interact with the electromagnetic field, which is the sense in which it is “dark”, while still imposing gravitational effects so that its presence can be deduced by gravitational lensing. Ultimately, gravitational interaction would be the primary way that dark matter could interact with the rest of the Universe, forming a massive part of its evolution due to its abundant presence.
Similarly, the existence of dark energy was deduced from redshift observations, primarily by using super novae of type Ia (SNIa) as a consistent reference point, albeit one limited by the accuracy of our measuring equipment. Based on the observed redshift and the energy that would be behind the needed acceleration, there should be more energy driving the expansion than we are aware of, or have so far measured as part of vacuum energy (also known as zero-point energy). This dark energy is calculated to form about 68.2% of the energy-matter balance within the Universe, with 26.8% being in the form of dark matter and a mere 5% ordinary matter.
Naturally, without more evidence and without reducing measurement error sizes, it’s hard to say whether any of this unseen energy and matter exists, is just a measurement error, or if the gravitational physics is just wrong. To this end we need instruments such as Euclid to refine and add to the available evidence.
Against this background, Euclid’s primary purpose is to continue the research on the exact nature of dark matter and energy by refining previous observations made of the shape of galaxies, as well as the redshift of objects like Type Ia supernovae. Earlier missions included the Planck space telescope (active from 2009 to 2013) and the Wilkinson Microwave Anisotropy Probe (WMAP, 2001 – 2010), with now Euclid and the James Webb Space Telescope (JWST) having taken over most of the observation tasks, with both based at the Lagrangian point L2 to keep them in the shade of the Earth.
The Euclid space telescope features visible light and near-infrared cameras, with both instruments being optimized for these specific observation tasks, whereas JWST is used for general observations in infrared. Although Euclid is currently still on its way to the L2 point before it can commence its science mission, JWST has been making relevant observations already, with some interesting data being published last year, such as last year’s findings on the redshift of very young galaxies (arXiv paper).
Both JWST and Euclid have near-infrared cameras, and can be considered to be complementary, but Euclid is solely dedicated to answering these specific questions in cosmology, allowing it to perform observations continuously and thus gather significant amounts of mission data over its projected six year lifespan.
What could be the implications of this research on dark matter and dark energy? Clearly, they are both essentially placeholders until we can ascertain whether our observations regarding the Universe’s expansion and the physics of galaxies can be explained in some other way. It wouldn’t be the first time over the past 120-odd years that our assumptions about how the Universe works and what its ultimate fate will be have fundamentally changed in the face of new evidence.
If the ΛCDM model holds up, then the question is whether we can define what the nature of dark matter and dark energy is. So far a number of hypotheses exist here, including sterile neutrinos and many more. For dark energy, the simplest explanation would be that it is an intrinsic energy of space (akin or equal to vacuum energy), yet many more options are possible. There is also the possibility that Euclid and other missions will provide us with data that puts into question many of the assumptions made, or leads us towards new possibilities.
Although none of what we learn via these observations is likely to change our fundamental understanding of the shape and rotation of galaxies, nor will it likely override Hubble’s law regarding the expansion of the Universe based on observations of other galaxies moving away from our own, what it will affect is our understanding of the how and why. Ultimately, by developing our elementary understanding of the energy-matter balance in the Universe and its exact nature we should have a much better idea of both how the Universe began, and how it will ultimately end.
Yet regardless of what we’ll discover, the essential part is that through Euclid, JWST and whichever spacecraft will follow them we’ll slowly untangle the biggest challenge for science, in the form of the Universe itself. This is a challenge that has kept the world’s greatest minds occupied for well over a century now, with the tantalizing promise of understanding the very foundations of existence itself.