News Excerpt:

There’s something wrong with our understanding of the Universe and how much it has expanded since the Big Bang.

Hubble tension:

• One of the biggest mysteries in cosmology is the ‘Hubble tension’.
• It is the puzzle that the expansion of the Universe we see today doesn’t match what we think it should be from looking at the early cosmos.
• The universe is expanding. How fast it does so is described by the so-called Hubble-Lemaitre constant. 
• However, there is a dispute about how big this constant actually is: Different measurement methods provide contradictory values. 
• The Hubble tension stems from the fact that measuring the Hubble-Lemaitre constant two different ways will get you two different answers. 

Big Bang and the Expansion of Universe:

• In the beginning, there was a Big Bang. 
• This massive explosion created the universe and filled it with the seeds of all the matter and energy currently present.
• But with that kind of turbo-kickstart, growth doesn’t end after the initial blast. 
• After the Big Bang, the universe continued to expand—even accelerating as it goes—at a rate referred to as the Hubble constant.

Lambda CDM:

• Today, the Lambda Cold Dark Matter (or ΛCDM) model is the latest development in our understanding of the origin of the Cosmos. 
• It represents an improvement of the Big Bang theory by positing that most of the physical substance in the Universe consists of a material called dark matter.
• Although it cannot be detected by current instrumentation, cosmologists believe dark matter is composed of cold, slow-moving particles that do not emit electromagnetic radiation or scatter light. Thus, they also appear dark. 
• However, the gravitational effect of dark matter can be observed on visible material, such as galaxies and observations of background radiation.
• It’s currently the simplest model that explains various features of the universe, including radiation leftover from the Big Bang, the arrangement of galaxies in the universe, and the fact that the universe is expanding.

Open, closed and flat universe:

• Our universe started to expand after the Big Bang event around 14 billion years ago. It may continue to expand unabated forever. 

Open Universe:

• If the universe continues to expand, it will be an open universe. 
• In an open universe, space will warp in the opposite direction. That is, it will have a negative curvature, resembling a saddle.

Closed Universe:

• If, at some point, the expansion of the universe stops because of the gravitational forces exerted by the galaxies, the universe could collapse and become closed.
• A closed universe is said to have a positive curvature of space – like a sphere. 
• Such a universe will be finite even if it has no bounds. 
• In this universe, we can travel forever without falling off an ‘edge’.

Flat Universe:

• Another possibility is that the universe will continue to expand forever, but the rate of expansion, which is currently increasing, will eventually start decreasing due to the gravitational forces. 
• The rate will take infinite time to drop to zero, so the universe will keep expanding, just slower and slower.
• This special approximation leads to a flat universe. And according to many cosmologists, this is the state of our universe at this time.
• That the universe is flat doesn’t mean it’s like a 2D sheet of paper. 
• Instead, flatness means that if you start to draw two parallel lines in space and keep drawing them, they will remain parallel no matter how far you go. (In a spherical or saddle-like space, the lines will intersect somewhere.)

Hubble versus James Webb Space Telescope data:

• The best way to follow the stars is by using the near-infrared radiation they emit. 
  • Unlike visible light, such radiation can pass through intervening dust clouds and reach us. 
• NASA’s James Webb Space Telescope (JWST) can track both near-infrared radiation and has instruments good enough to distinguish between radiation from two Cepheid variable stars close to each other in the sky.
• In a study published in The Astrophysical Journal Letters, researchers examined the concern that the data collected by NASA’s previously best space telescope, the Hubble, had some flaws in its readings, which gave rise to the Hubble tension.
• They analysed over a thousand sharp observations of Cepheid variables recorded by JWST. 
  • “The superior resolution of JWST negates crowding noise, the largest source of variance in the near-infrared [brightness] relations measured with the Hubble space telescope,” they wrote.
• In the end, they found “no significant difference” in estimates of the stars’ distance based on Hubble telescope and JWST data, even after correcting for “local crowding” and “choice of filters”.
• The Hubble tension is real, and its origins remain a mystery.