Cosmological Models of the Universe

Coralie Nettles, Artem Cheprasov
  • Author
    Coralie Nettles

    Coralie has taught university physics and tutored high school and college students in STEM since 2012. She has a bachelor of science and doctorate in physics from the University of Newcastle. She has worked at universities and schools in three different countries.

  • Instructor
    Artem Cheprasov

    Artem has a doctor of veterinary medicine degree.

Learn whether the universe is open, closed, or flat by examining cosmological models. Understand the different cosmological models and why a flat universe has strong evidence. Updated: 12/06/2021

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Is The Universe Flat?

An important question in cosmology is, is space flat? Cosmology is the study of the history and future of the universe. One branch of cosmology, physical cosmology, studies the origin, structure, and fate of the universe. This study draws on work from many areas of both theoretical and experimental physics.

In cosmology, when people refer to the shape of the universe, they are generally referring to the curvature. Specifically, is the universe flat or curved? If the universe is curved, is it open or closed? There are three main cosmological models that have been described for the shape of the universe. These are:

  • A flat universe with zero curvature.
  • An open universe with a curvature that does not curve back on itself.
  • A closed universe with a curvature that folds back on itself like a saddle.

According to the Theory of General Relatively developed by German-born theoretical physicist Albert Einstein, the overall shape of the universe depends on the average density of matter and energy in the universe. The universe can only be flat if the density of the matter and energy precisely equals a value known as the critical density. A density parameter is also defined as the ratio between the actual density of the universe and the critical density. The parameter is represented by {eq}\Omega {/eq}.

  • For a flat universe: {eq}\Omega = 1 {/eq}.
  • For a closed universe: {eq}\Omega > 1 {/eq}
  • For an open universe:{eq}\Omega < 1 {/eq}.

The density parameter has been calculated experimentally in a number of ways. These experiments then provide evidence for theories regarding the shape of the universe.

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Flat Universe

The exact shape is still up for debate; however, experimental data from many independent sources indicate that the universe is most likely flat. The flat universe model can be visualized as a sheet of paper, with no curvature. In this model, the universe is infinite and will continue to expand forever.

Diagram showing the structure of a flat universe.

flat plane

The density parameter has been determined experimentally in several ways. One method is to sum all the mass energy in the universe and find the average density. This has been done using data from the Wilkinson Microwave Anisotropy Probe (WMAP) and Planck spacecraft which performed measurements of the Cosmic Microwave Background radiation. Another method is to measure the angle across the observable universe, which is also done using the CMB. This method was used in the BOOMERanG experiment using high-altitude balloons. These measurements have all returned values of the density parameter that correspond to a flat universe within experimental error.

Why is the universe flat? This is believed to be a consequence of inflation. In cosmology, inflation describes the theory that the universe underwent an exponential expansion of space the instant after the Big Bang. The expansion only lasted for {eq}10^{-36} {/eq} seconds after the Big Bang before the expansion slowed significantly. Experimental evidence from as early as the 1960s began to indicate that the density of the universe was very close to the critical density, or the density necessary for a flat universe. Inflation can explain why the overall shape of the universe is flat or very close to flat. Consider a point on a balloon as it is inflated, the surface around a single point flattens as the balloon inflates. The flattening of space is thought to have occurred in the same way and would have been so enormous that any curvature that the universe had previously would only be noticeable on scales much larger than the observable universe.

Closed Universe

Another theory about the structure of the universe is the closed universe theory. So, what is the meaning of a closed universe? A closed universe describes one with non-zero curvature which curves back on itself. This can be thought of as a sphere, as shown in the diagram. A closed universe is said to have positive curvature.

Diagram showing the spherical shape of a closed universe.

closed universe sphere

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Frequently Asked Questions

What happens in a closed universe?

For a closed universe, the expansion will eventually be stopped by the force of gravity. After this, the universe will begin to collapse to a single point.

What is an open universe?

An open universe is one that is curved by does not fold back onto itself. This type of universe will expand forever because there is not enough mass for gravity to stop it.

Is the universe geometrically flat?

The exact shape of the universe is still up for debate. But the best current theories suggest that the universe is geometrically flat.

What happens if the universe is closed?

In a closed universe, gravity will eventually stop the universe from expanding. After this, it will begin to contract to a single point called the Big Crunch.

Is the universe closed, open, or flat?

There is not a general consensus on the shape or structure of the universe. Several theories exist which each agrees with some available evidence. The best current theories predict the universe is likely flat.

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