How Fast is The Universe Expanding? Understanding Hubble Tension

We know the universe is expanding but results of calculations can't seem to agree on the speed. The values are too inconsistent to ignore, and it may be pointing to something missing from our understanding of the cosmos.

Understanding Hubble's Law

Hubble's Law observes that galaxies moving away from earth move at speeds proportional to their distance from earth. The speed at which a galaxy moves away from earth is known as its recessional velocity. This quantity can be determined by measuring the red shift of a galaxy: the increase in electromagnetic wavelength of the light emitted by the galaxy (similar to how a siren sounds lower in pitch as it moves away from you).

What is the relationship between the distance of a galaxy and its recessional velocity? The estimate of the value of their ratio is called the Hubble Constant. Determining the Hubble Constant allows us to calculate the speed of the universe's expansion and its age.

Measuring the Hubble Constant

Astronomers first looked at relatively close cosmic objects. To figure out how far away from us each of these objects were, they used a succession of methods called the cosmic distance ladder:

• Parallax: Calculating the distance using trigonometry by first measuring the 'shift' in the object's position at different points in the earth's orbit around the sun.

• Standard Candles: Calculating distance by comparing the observed brightness of an object to its true luminosity and using the inverse-square law of light.

• Type Ia Supernovae: Using Type Ia supernovae as anchors because they explode quite similarly, reaching nearly the same peak brightness each time.

  
  

Using telescopes, such as Hubble and JWST, astronomers calculated an expansion speed of around 73 km/s/Mpc.

Another way of measuring the Hubble Constant is to use the Cosmic Microwave Background (CMB), the leftover radiation from the early universe. Observatories like ESA's Planck Satellite map the light patterns. This data can then be plugged into something called the Standard Model of Cosmology: a complicated model that links three main components of our universe (ordinary matter, cold dark matter, dark energy). This calculation results in a Hubble constant value of 67.4 km/s/Mpc.

Defining the Problem of Hubble Tension

Notice how both the calculated values for Hubble Constant are different? (73 km/s/Mpc and 67.4 km/s/Mpc) Although the numbers themselves may not seem so far apart, such a difference is unexpected considering the precision of the instruments involved in making these calculations. This discrepancy between calculated values of the Hubble Constant is called the Hubble Tension.

The initial suspicion was that the telescopes had some sort of error. This doubt was resolved when the JWST was used to double check values from local stars, confirming the calculated figure to be correct. If both values are correct, it would mean that there is something fundamental missing in the model of our universe. What could possibly be missing in our physics is the exact dilemma posed to astrophysicists everywhere today.

What This Means For Physics

There could be a plethora of reasons why the discrepancy is happening, but three main reasons are widely discussed. First, it could stem from some sort of systematic error affecting all observations. However, this is increasingly considered unlikely (though not entirely ruled out) because the error would have to occur on all independent instruments used and there are no obvious candidates for what the error could be. Second, our assumptions about the universe may be incomplete, which is closely related to a third possibility: that the discrepancy is due to entirely new physics that goes beyond the standard model. There are many theories in this category which involve a modified theory of gravity, early dark energy or decaying dark matter. If this is true, further studies will have to be conducted to test our current cosmological model. Accepting any of the proposed theories would mean it would modify our current cosmological model. However, doing so without altering our present and accurate understandings remains difficult.