Written by: Maneth Perera

Everyday, new evidence regarding the structure of our universe is uncovered, leading to researchers revising mathematical models over decades to perfectly describe their observations. The widely accepted foundational model of our universe is called the Lambda Cold Dark Matter (CDM) model, named for its focus on dark matter and dark energy to make up for observational discrepancies. However, recent studies have added to our observations, and now researchers find that certain foundational elements of the Lambda CDM model can be disproven by a newer model. Instead of generalizing the forces of the universe, by reframing spacetime as a “lumpy” formation, we can use changes in time to better explain our observations so far.

Figure 1

A graphic showing the expansion of the universe over time. The curved part at the end is meant to show the universe’s acceleration and its increase in growth rate.

Source: Royal Astronomical Society

Accelerating Expansions

In 1922, Alexander Friedmann built upon Albert Einstein’s previous mathematical equations that modeled the universe. Einstein’s theories of special relativity (how light travels through space and time) and general relativity (how time, space, and gravity are connected) allowed for astronomers to use physics to map out the universe. An important clue they found was that the universe obeyed the same laws of physics in every location, and thus if it was expanding, it would expand at the same rate in each place. Friedmann showed that the universe was expanding through Einstein’s work, but much later in 1929, Edwin Hubble found that the universe was in fact accelerating in its expansion. Through measuring the speed at which galaxies move away from us, Hubble found that the farther away a galaxy is from us, the faster it would move. (NASA, n.d.) Imagine drawing dots on a balloon; if you blow the balloon up, those dots would start off moving away from each other slowly before moving faster and faster.

In order for the universe to be accelerating in its expansion, it must have some force that pushes outwards. Gravity acts between every object in the universe due to its infinite range (although it’s extremely weak unless the objects are close and have large masses, which is why we only feel the Earth’s gravity). Some force had to be moving against gravity to create an accelerating expansion rate, and thus researchers proposed an undetectable form of energy called “dark energy” that caused this growth.

Figure 2

A depiction of different galaxies’ movements over time, curving away from our vantage point.

Source: Fiveable

Voids versus Galaxies

The timescape model differs from other models as it doesn’t assume that the universe expands the same in every location. The model postulates that the amount of matter in a given location will have a large gravitational pull. Therefore, by the theory of general relativity, time will go slower in that area. In contrast, voids between galaxies where there is very little matter and low gravity will have their time speed up. This difference in time means that a constant expansion rate in all areas of the universe (without dark energy and with acceleration at 0) would cause voids to grow faster due to the extra time being experienced. Researchers found that with this model, time in the Milky Way galaxy would be approximately 35% slower than time in these matter-less voids, meaning that these voids would experience billions of more years of time when taking into account how long the universe has existed. (Seifert et. al, 2024) This new viewpoint offers a proper explanation as to why the universe’s expansion seems to be accelerating, and is backed up by new high-precision data from the Dark Energy Spectroscopic Instrument experiments. (Seifert et. al, 2024)

Figure 3

A depiction of Earth curving the fabric of spacetime around it, as any piece of matter has a gravitational pull on all scales.

Source: Wikimedia Commons

Strengths and Weaknesses of the Model

One of the main drawbacks of the widely accepted CDM model was the fact that it assumed the universe will have the same force pushing against gravity throughout without considering how time affects expansion. Previously, researchers have tried to explain this using boosts of velocity caused by special relativistic effects. (Seifert et. al, 2024) These local boosts are attributed to our changing perspective of the universe influencing our observations (due to the solar system’s movement in the Milky Way), but a much more likely theory is that expansion is simply not uniform due to time, which ties in with what we already know from general relativity.

Although the timescape model makes up for these discrepancies in other models and allows for a more complete picture, it is still a relatively new model and doesn’t have too much experimental evidence. (Royal Astronomical Society, 2024) The timescape model is currently being investigated using type 1A supernova data. The European Space Agency’s Euclid satellite, similar to the Dark Energy Spectroscopic Instrument, has been cataloging thousands of supernovas to investigate how quickly they move away from us and the size of the voids between them. The model fits nicely with much of our old data, but until we get proper readings that are specifically meant to test it, we can’t be sure if it will completely replace our current understanding.

Conclusion

Although we are far from a conclusive determination of why exactly the universe grows the way it does, the revision of old astrophysical laws shows us how far we have come when studying the cosmos. The Lambda CDM model and others still have their merits, but many scientists expect parts of them to be disproved or at least revised in the coming decades as new data is found. In fact, the timescape model may also be heavily modified as we find out more through new observations and new mathematical models.

References and Sources

Gelderman, R. (2014). Cosmological Principle. Western Kentucky University. https://astro.wku.edu/astr106/structure/cosmologicalprinciple.html

NASA. (n.d.). Measuring the Universe’s Expansion Rate. HubbleSite. Retrieved January 11, 2025, from https://hubblesite.org/mission-and-telescope/hubble-30th-anniversary/hubbles-exciting-universe/measuring-the-universes-expansion-rate

Royal Astronomical Society. (2024, December 19). Dark energy “doesn’t exist” so can’t be pushing “lumpy” Universe apart – study. The Royal Astronomical Society. https://ras.ac.uk/news-and-press/research-highlights/dark-energy-doesnt-exist-so-cant-be-pushing-lumpy-universe-apart

Seifert, A., Lane, Z. G., Galoppo, M., Ridden-Harper, R., & Wiltshire, D. L. (2024). Supernovae evidence for foundational change to cosmological models. Monthly Notices of the Royal Astronomical Society: Letters, 537(1), L55–L60. https://doi.org/10.1093/mnrasl/slae112