Sky & Telescope - March 2022 - 15
GURGEN BAKHSHETYAN / SHUTTERSTOCK.COM
T
he current theory of how the universe works is called
the lambda cold dark matter (ΛCDM) model. It starts
with the Big Bang, assumes that Einstein's general
theory of relativity (GR) is just fine, and posits that the universe
is composed primarily of three things: (1) dark energy,
a repulsive pressure described by the constant Lambda (Λ);
(2) cold dark matter (CDM); and (3) ordinary matter.
People call ΛCDM the " standard model " of cosmology
because it is relatively simple and makes good sense of phenomena
such as the cosmic microwave background (CMB),
the way galaxies cluster, and much more. It has been around
since the 1980s, even before two independent teams confirmed
in 1998 that the expansion of the universe is accelerating
due to dark energy, not slowing down as expected. ΛCDM
is a good working model, and most astronomers accept it.
Which makes it a bit awkward that different methods of
measuring the universe's current expansion rate, called the
Hubble constant or H0 (pronounced " H-naught " ), don't match.
The issue is between two categories of measurements, one
more direct than the other (S&T: June 2019, p. 22). The problem
even has a name: the Hubble tension. Wendy Freedman
(now at University of Chicago) and her colleagues reported
the first reasonably precise direct measurements of the current
expansion rate in 2001 (yielding a value of about 72 km per
second per megaparsec, with an uncertainty of 11%). Then the
Planck team announced the first high-precision indirect measurements
in 2013 (about 67 km/s/Mpc, with an uncertainty
of less than 2%). Many cosmologists assumed that these two
very different approaches would eventually converge on the
same number - but so far they haven't.
A Cosmic Rewind
The traditional way of measuring H0 involves a cosmic distance
ladder, in which astronomers use a variety of methods to meask
yand tele scope .o r g * MARCH 2022 15
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Sky & Telescope - March 2022
Table of Contents for the Digital Edition of Sky & Telescope - March 2022
Contents
Sky & Telescope - March 2022 - Cover1
Sky & Telescope - March 2022 - Cover2
Sky & Telescope - March 2022 - 1
Sky & Telescope - March 2022 - Contents
Sky & Telescope - March 2022 - 3
Sky & Telescope - March 2022 - 4
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Sky & Telescope - March 2022 - Cover3
Sky & Telescope - March 2022 - Cover4
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