The universe keeps expanding. When researchers analyze data from the Hubble and James Webb telescopes, alongside a suite of other astronomical tools, they find that the recessional velocity of galaxies, the speed at which they appear to move away from the Earth, varies depending on what they measure.

If they calibrate distances deep into the cosmos using Cepheid variable stars, the expansion rate appears faster than when they use red giant stars or the Cosmic Microwave Background (CMB). This discrepancy, known as the Hubble tension, reveals a deeper mystery: different cosmic yardsticks yield different rates of expansion.

Yet despite the disagreement in values, all methods affirm the same truth: space is stretching…a lot…like a sheet pulled and stretched taut between Atlas’s burden and Hermes flight: a cosmos caught between gravitational pull and a mysterious push: Pushmi-Pullyu on a cosmic scale.

To understand why the cosmos resembles a sheet of rubber we need to travel back about 110 years and peer into the minds of those who first saw increasing separation as a universal law. These new architects of reality: Einstein, Friedmann, Lemaitre; who replaced Newton’s planetary, static models of the cosmos with a dynamic spacetime of bends, ripples, and persistent expansion.

After Einstein published his General Theory of Relativity in 1915, Russian physicist Alexander Friedmann’s analysis of his work showed that the universe could be expanding, and that Einstein’s equations could be used to calculate the rate. In 1927 Belgium priest and physicist Georges Lemaitre proposed that the expansion might be proportional to a galaxy’s velocity relative to its distance from Earth. By 1929, American astronomer Edwin Hubble expanded on Lemaitre’s work and published what became known as Hubble-Lemaitre law: galaxies are moving away from us at speeds proportional to their distance. The greater the distance the faster the speed.

A key feature of this law is the Hubble constant, the proportionality that links velocity and distance. Hubble’s initial estimate for this constant was whopping, and egregiously off, 500 kilometers per second per megaparsec (km/s/Mpc), but as measurements improved, it coalesced around a range between 67 and 73, with the most recent value at 70.4 km/s/Mpc, published by Freedman et al. in May 2025.

The Hubble constant is expressed in kilometers per second per megaparsec. The scale of these units is beyond human comprehension but let’s ground it to something manageable. A megaparsec is about 3.26 million light-years across, and the observable universe, though only 13.8 billion light-years old, has stretched to 46 billion light-years in radius, or 93 billion light-years in diameter, due to the expansion of space (see mind warping explanation below).  

To calculate the recessional velocity across this vast distance, we first convert 46 billion light-years into megaparsecs: which equates to 14,110 megaparsecs. Applying Hubble’s Law: 70 km/s/Mpc times 14,110 Mpc equals 987,700 km/s. This is the rate at which a galaxy 46 billion light-years away would be receding relative to another galaxy one megaparsec closer to Earth.

That’s more than three times the speed of light (299,792 km/sec) or Warp 3 plus in Star Trek parlance. Einstein said this was impossible but fortunately there is some nuance that keeps us in compliance with Special Relativity (or else the fines would be astronomical). This isn’t the speed of a galaxy moving through space, but the speed at which space between galaxies is expanding. Which, admittedly, is terribly confusing.

The speed of a galaxy, composed of matter, energy, and dark matter, must obey Einstein’s rules: gravity and Special Relativity. And one of the rules is that the speed of light is the cosmic speed limit, no one shall pass beyond this.

But space between the galaxies decides to emphasize the rules in a different order. The expansion of space is still governed by Einstein’s equations, just interpreted through the lens of spacetime geometry rather than the motion of objects. This geometry is shaped by, yet not reducible to, matter, energy, and dark matter.

Expansion is a feature of spacetime’s structure, not velocity in the usual sense, and thus isn’t bound by the speed of light. If space wants to expand, stretch, faster than a photon can travel, well so be it.

The space between galaxies is governed by dark energy and its enigmatic rules of geometry. Within galaxies, the rules are set by dark matter, and to a lesser extent by matter and energy, even though dark energy is likely present, its influence at galactic scales is minimal.

Note the use of the word scale here. Galaxies are gigantic, the Milky Way is 100,000-120,000 light-years in diameter. But compared to the universe at 93,000,000,000 light-years across, they’re puny. You would need 845,000 Milky Ways lined up edge-to-edge to span the known universe.

Estimates of the number of galaxies in the universe range from 100 billion to 2 trillion. So, at the scale of the universe, galaxies are mere pinpoints of light; blips of energy scattered across the ever-expanding heavens.

This brings us to dark energy, the mysterious force driving cosmic expansion. No one knows what it is, but perhaps empty space and dark energy are the same. There’s even some speculation, mostly mine, that dark energy is a phase shift of dark matter. A shift in state. A triptych move from Newtonian physics to Quantum Mechanics to…Space Truckin’.

In the beginning moments after the big bang, the universe was dominated by radiation composed of high energy particles and photons. As the universe cooled, the radiation gave way to matter and dark matter. As more time allowed gravity to create structures, black holes emerged and a new force began to dominate, dark energy. But where did the dark energy come from? Was it always part of the universe or did it evolve from other building blocks. Below are a few speculative ideas floating around the cosmic playroom.

J.S. Farnes proposed a unifying theory where dark matter and dark energy are aspects of a single negative mass fluid. This fluid could flatten galaxy rotation curves and drive cosmic expansion, mimicking both phenomena simultaneously.

Mathematicians Tian Ma and Shouhong Wang developed a unified theory that alters Einstein’s field equations to account for a new scalar potential field. Their model suggests that energy and momentum conservation only holds when normal matter, dark matter, and dark energy are considered together.

Ding-Yu Chung proposed a model where dark energy, dark matter, and baryonic matter emerge from a dual universe structure involving positive and negative mass domains. These domains oscillate and transmute across dimensions.

These ideas all rotate around the idea that reality revolves around a concept that everything evolves and that matter and energy, of all forms, flickers in and out of existence depending on dimensional scaffolding of space and the strength of gravity and radiation fields.  Rather than radiation, energy, matter, dark matter, and dark energy as separate entities, these may be expressions of a single evolving field, shaped by phase transitions, scalar dynamics, or symmetry breaking.

Now back to my regularly scheduled program. In August 2025, Quanta Magazine reported on a study led by Nobel laureate Adam Riess using the James Webb Telescope (JWST) to measure over 1,000 Cepheid variable stars with unprecedented precision. Cepheid stars pulsate in brightness over time with a highly predictable rate or rhythm, making them ideal cosmic yardsticks. Riess’s team found a Hubble constant of ~73.4 km/s/Mpc, consistent with previous Hubble Space Telescope measurements of Cepheid stars but still significantly higher than what theory predicts.

That theory comes from the standard model of cosmology: Lambda Cold Dark Matter. According to this framework photons decoupled from the hot electron-proton opaque soup about 380,000 years after the Big Bang went boom, allowing light to travel freely for the first time, and allowing space to be somewhat transparent and visible. This event produced the Cosmic Microwave Background (CMB).

This CMB permeates the universe to this day. It was discovered in 1964 by Bell Lab physicists Arno Penzias and Robert Wilson, who were trying to eliminate background noise from their radio antenna. The noise turned out to be the faint afterglow from the Big Bang, cooled down from its original 3000 Kelvin to a frosty 2.7 Kelvin. They received the Nobel Prize in Physics for this discovery in 1978.

Light from the CMB, as measured by the European Space Agency Planck satellite, has a redshift of approximately 1100, meaning the universe has expanded by a factor of 1100 over the past 13.42 billion years. By analyzing the minute temperature fluctuations in the CMB, Planck can infer the density of matter, dark energy, and curvature of the universe. Inserting these parameters into the Lambda Cold Dark Matter model yields a Hubble constant which turns out to be 67.4 + 1.71 (65.69-69.11). This value is considered the gold standard. Values beyond the Planck measurement are not necessarily wrong, just not understood.

At first glance, the difference between Planck’s 67.4 and Riess’ 73.4 may seem small. But it is cosmically significant. Two galaxies 43 billion light-years away and 3.26 billion light-years apart (1000 Mpc) would have a velocity difference of 6000 km/s or about 189 billion kilometers of increased separation per year. That’s the scale of what small differences in the value can add up to and is referred to as the Hubble tension.

Meanwhile, a competing team of researchers studying red branch and giant branch stars consistently scored the Hubble constant closer to the theoretical prediction of 67.4. This team led by Wendy Freedman believes that Hubble tension, the inability of various methods of measuring the Hubble constant to collapse to a single value, is a result of measurement errors

While some researchers, Wendy Freedman and others, suggest lingering systematic errors may still be at play, the persistence of this discrepancy, across instruments, methods, and team, has led others to speculate about new physics. Among the most provocative ideas: the possibility that the universe’s expansion rate may vary depending on direction, hinting at anisotropic expansion and challenging the long-held assumption of cosmic isotropy. But this seems far-fetched and if true it would likely break the Lambda Cold Dark Matter model into pieces.

And so, the cosmos grows lonelier. Not because the galaxies are fleeing, but because space itself is stretching, a wedge governed by the geometry of expansion. The further they drift apart, the less they interact, a divorce from neglect rather than malice. In time, entire galaxies will slip beyond our cosmic horizon, receding faster than light, unreachable even in principle. A cosmos of the lonely.

Source: The Webb Telescope Further Deepens the Biggest Controversy in Cosmology by Liz Kruesi, Quanta Magazine, 13 August 2024. JWST Observations Reject Unrecognized Crowding of Cepheid Photometry as an Explanation for the Hubble Tension at 8σ Confidence by Riess et al, The Astrophysical Journal Letters, 6 February 2024. Graphic: Cosmic Nebula by Margarita Balashova.