7.5 KiB
| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| Age of the universe | 3/4 | https://en.wikipedia.org/wiki/Age_of_the_universe | reference | science, encyclopedia | 2026-05-05T13:31:45.824475+00:00 | kb-cron |
where
H
0
{\displaystyle ~H_{0}~}
is the Hubble parameter and the function
F
{\displaystyle ~F~}
depends only on the fractional contribution to the universe's energy content that comes from various components. The first observation that one can make from this formula is that it is the Hubble parameter that controls that age of the universe, with a correction arising from the matter and energy content. So a rough estimate of the age of the universe comes from the Hubble time, the inverse of the Hubble parameter. With a value for
H
0
{\displaystyle ~H_{0}~}
around 69 km/s/Mpc, the Hubble time evaluates to
1
/
H
0
=
{\displaystyle ~1/H_{0}=~}
14.5 billion years. To get a more accurate number, the correction function
F
{\displaystyle ~F~}
must be computed. In general this must be done numerically, and the results for a range of cosmological parameter values are shown in the figure. For the Planck values
(
Ω
m
,
Ω
Λ
)
=
{\displaystyle ~(\Omega _{\text{m}},\Omega _{\Lambda })=~}
(0.3086, 0.6914), shown by the box in the upper left corner of the figure, this correction factor is about
F
=
0.956
.
{\displaystyle ~F=0.956~.}
For a flat universe without any cosmological constant, shown by the star in the lower right corner,
F
=
2
/
3
{\displaystyle ~F={2}/{3}~}
is much smaller and thus the universe is younger for a fixed value of the Hubble parameter. To make this figure,
Ω
r
{\displaystyle ~\Omega _{\text{r}}~}
is held constant (roughly equivalent to holding the cosmic microwave background temperature constant) and the curvature density parameter is fixed by the value of the other three. Apart from the Planck satellite, the Wilkinson Microwave Anisotropy Probe (WMAP) was instrumental in establishing an accurate age of the universe, though other measurements must be folded in to gain an accurate number. CMB measurements are very good at constraining the matter content
Ω
m
,
{\displaystyle ~\Omega _{\text{m}}~,}
and curvature parameter
Ω
k
.
{\displaystyle ~\Omega _{\text{k}}~.}
It is not as sensitive to
Ω
Λ
{\displaystyle ~\Omega _{\Lambda }~}
directly, partly because the cosmological constant becomes important only at low redshift. The most accurate determinations of the Hubble parameter
H
0
{\displaystyle ~H_{0}~}
are believed to come from measured brightnesses and redshifts of distant Type Ia supernovae. Combining these measurements leads to the generally accepted value for the age of the universe quoted above. The cosmological constant makes the universe "older" for fixed values of the other parameters. This is significant, since before the cosmological constant became generally accepted, the Big Bang model had difficulty explaining why globular clusters in the Milky Way appeared to be far older than the age of the universe as calculated from the Hubble parameter and a matter-only universe. Introducing the cosmological constant allows the universe to be older than these clusters, as well as explaining other features that the matter-only cosmological model could not.
== Lookback time == Light observed from astronomical objects was emitted when the universe was younger. Astronomers use lookback time,
t
L
{\displaystyle t_{L}}
, to describe the difference in the age of the universe here and now,
t
0
{\displaystyle t_{0}}
, from the age at the time of emission,
t
e
{\displaystyle t_{e}}
:
t
L
(
z
)
=
t
0
−
t
e
(
z
)
{\displaystyle t_{L}(z)=t_{0}-t_{e}(z)}
where t is an age. The lookback time depends on the object's redshift and, like the age of the universe, the cosmological parameters selected.
== WMAP == NASA's Wilkinson Microwave Anisotropy Probe (WMAP) project's nine-year data release in 2012 estimated the age of the universe to be (13.772±0.059)×109 years (13.772 billion years, with an uncertainty of plus or minus 59 million years). This age is based on the assumption that the project's underlying model is correct; other methods of estimating the age of the universe could give different ages. Assuming an extra background of relativistic particles, for example, can enlarge the error bars of the WMAP constraint by one order of magnitude. This measurement is made by using the location of the first acoustic peak in the microwave background power spectrum to determine the size of the decoupling surface (size of the universe at the time of recombination). The light travel time to this surface (depending on the geometry used) yields a reliable age for the universe. Assuming the validity of the models used to determine this age, the residual accuracy yields a margin of error near one per cent.
== Planck == In 2015, the Planck Collaboration estimated the age of the universe to be 13.813±0.038 billion years, slightly higher but within the uncertainties of the earlier number derived from the WMAP data.
In the table below, figures are within 68% confidence limits for the base ΛCDM model.
Legend:
TT, TE, EE: Planck Cosmic microwave background (CMB) power spectra lowP: Planck polarization data in the low-ℓ likelihood lensing: CMB lensing reconstruction ext: External data (BAO+JLA+H0). BAO: Baryon acoustic oscillations, JLA: Joint Light curve Analysis, H0: Hubble constant
In 2018, the Planck Collaboration updated its estimate for the age of the universe to 13.787±0.020 billion years.