Recommended Free Tools
Scientists estimate the universe’s present-day expansion rate, called the Hubble constant (H0), by comparing how far away galaxies are with how much their light has shifted toward longer wavelengths. One method builds distances outward from nearby stars; another infers today’s rate from the early-universe cosmic microwave background using a cosmological model. These approaches currently give different answers, and the discrepancy—the Hubble tension—has not been resolved.
What the Hubble constant measures
The Hubble constant describes the present-day relationship between a galaxy’s distance and the rate at which the universe’s expansion carries it away from us. It is commonly expressed in kilometres per second per megaparsec (km/s/Mpc): a megaparsec is a large unit of distance, and the units express how much the expansion-related recession rate changes with distance.
As an Amazon Associate I earn from qualifying purchases.
A galaxy’s redshift is the stretching of its light toward longer wavelengths as the universe expands. Astronomers combine redshift information with a distance scale to estimate H0. This is not a claim that every galaxy moves at the same speed: galaxies at different distances have different expansion-related recession rates, and local motions can also affect observations.
How the cosmic distance ladder works
The local distance ladder links several kinds of observations. Each rung is calibrated using the one before it, so the result depends on a chain of measurements rather than a single instrument or reading.
#1 Best Overall
- 【Excellent Optics】Astronomical telescope features 80mm aperture 600mm(f/6.7) focal length-The big aperture can capture more light; high transmission multi-fully coated optical lens can improve light transmittance, reduce the reflection of light. It will bring you more bright and clear images.
- 【Portable & Stable】Our refractor telescope has a backbag, an adjustable aluminum tripod and an upgraded phone adapter. All accessories can be packed into the bag, which is convenient for you to carry and storage for traveling. The tripod is stable and the height can adjust from 17.7" to 52", which is suitable for adults and kids. With the phone adapter, you can take amazing photos through your phone.
- 【Easy to Operate】 This telescope is easy to assemble even for astronomy beginners and kids. You can set up easily with the detail installation manual and video. No tools are required. It’s a great astronomy gift telescope for adults, astronomy enthusiasts, beginners and kids 8-12.
- 【Optimum Magnification】Our telescope for kids and adults is equipped with two replaceable excellent-quality eyepiecesa (20mm and 9mm) for 30X and 66X magnification. A moon filter transmits only 13 percent of the incoming light and improving contrast. Also with 5x24 finderscope, you can locate objects easily. A great option for astronomy lovers to explore the moon, Saturn and Jupiter.
- 【Satisfaction Service】We provide 3 years satisfaction service. If you have any questions about the product and service, please feel free to contact us to get 24-hour technical support from our expert team. Whether observing moon or viewing planets, our telescope can meet your all needs. It’s a great telescope for adults & 8-12 kids & astronomy beginners.
- Measure nearby stars geometrically. Parallax is the apparent change in a star’s position as Earth’s viewpoint changes during its orbit. For nearby stars, this provides a geometric distance that can calibrate other distance indicators. NASA’s “Hubble Constant and Tension” explainer describes this as the starting point of the ladder.
- Calibrate Cepheid variable stars. Cepheids brighten and dim in a regular cycle. Their pulsation properties are related to their intrinsic brightness. Once that relationship is calibrated using stars with known distances, astronomers can compare a Cepheid’s intrinsic brightness with how bright it appears to estimate its distance in another galaxy.
- Use Cepheids to calibrate Type Ia supernovae. Some galaxies that contain Cepheids also host Type Ia supernovae. Because these stellar explosions are much brighter, they can be observed in more distant galaxies. Astronomers standardize their luminosities, using galaxies with Cepheid-based distances as calibrators.
- Extend the distance scale and compare with redshift. Standardized Type Ia supernovae in more distant galaxies extend the distance measurements. Comparing those distances with the galaxies’ redshifts allows astronomers to estimate the present-day expansion rate.
The links in this chain are not perfect. Calibration uncertainty, dust, crowded fields of stars, and differences in supernova properties can affect the result. The Particle Data Group’s 2024 review notes that Type Ia supernovae are not perfect standard candles and discusses possible effects such as extinction and redshift evolution.
How the cosmic microwave background gives a different estimate
The cosmic microwave background (CMB) is radiation from the early universe. Instead of measuring distances to nearby stars and then building outward, scientists fit the CMB’s observed patterns with a cosmological model, commonly ΛCDM, and use that fit to infer what the expansion rate should be today.
Rank #2
- Superior Optics: 400mm(f/5.7) focal length and 70mm aperture, fully coated optics glass lens with high transmission coatings creates stunning images and protect your eyes. Perfect full positive telescope for astronomers to explore stars and moon.
- Magnification: Come with two replaceable eyepieces and one 3x Barlow lens.3x Barlow lens trebles the magnifying power of each eyepiece. 5x24 finder scope with mounting bracket and cross-hair lines inside make locating objects easily.
- Wireless Remote: This refractor telescope includes one smart phone adapter and one Wireless camera remote to explore the nature of the world easily through the screen and take amazing celestial images.
- Adjustable Tripod: This telescope allows for many different viewing positions with a adjustable aluminum alloy tripod and a carry bag, the telescope and tripod can fit inside the bag for easy traveling and storage.
- Satisfaction: Buy with confidence from a leading manufacturer.
That is a model-based inference, not a direct measurement of today’s H0. NASA LAMBDA’s “Hubble Constant” overview emphasizes that the CMB is a high-redshift snapshot; the model is used to calculate the value at redshift zero. The inference also depends on the model’s parameters. The Particle Data Group’s 2024 review describes degeneracies involving parameters including matter density and neutrino mass.
Outdated Drivers Are Slowing You Down
One free scan finds every outdated or missing driver and matches the right update for your exact hardware.Free scan · exact hardware matchPC Slower Than It Used to Be?
A free scan shows the junk files, broken settings and background clutter dragging Windows down - then fixes them in one click.Free scan · Windows 10 & 11Why published values differ
The figures below are representative results cited by the Particle Data Group’s 2024 “Cosmological Parameters” review, published 31 May 2024. They are not a complete 2026 catalogue or a single agreed value. The estimates use different observations and assumptions, so their uncertainties should be read with their methods and publication vintage.
Rank #3
- SMARTPHONE-POWERED SKY TOUR: No experience needed! Just dock your phone, launch the StarSense Explorer app, and follow the on-screen arrows to locate stars, planets, nebulae, and more.
- PATENTED STARSENSE TECHNOLOGY: Unlike other astronomy apps, StarSense Explorer uses sky recognition technology to turn your phone into a celestial navigation system, analyzing star patterns overhead to pinpoint your telescope’s position.
- TONIGHT’S BEST TARGETS, INSTANTLY: The app generates a curated list of the top objects to see based on your time and location. See planets, bright nebulae, galaxies, and star clusters from the city—and even more from dark skies.
- SIMPLE SETUP, SMOOTH TRACKING: Features a manual altazimuth mount with altitude slow motion adjustment with a sliding rod. Follow the on-screen arrows to your target; when the bullseye turns green, you can view it clearly through the eyepiece.
- 114MM REFLECTOR WITH IMPRESSIVE VIEWS: The 4.5" Newtonian reflector with high-reflectivity coatings delivers sharp, vivid views of the Moon, planets like Jupiter and Saturn, and deep-sky favorites like the Orion Nebula and Andromeda Galaxy.
| Estimate | Method and qualification | Source and vintage |
|---|---|---|
| 73.0 ± 1.0 km/s/Mpc | SH0ES local distance ladder; the cited analysis used Hubble Space Telescope Cepheids in hosts of 42 Type Ia supernovae and Gaia EDR3 parallaxes. | Particle Data Group review, 2024, summarizing the SH0ES 2022 result. |
| 69.8 ± 0.6 (statistical) ± 1.6 (systematic) km/s/Mpc | Carnegie–Chicago Hubble Program (CCHP); calibrates Type Ia supernovae with the tip of the red-giant branch. | Particle Data Group review, 2024. |
| 67.4 ± 0.5 km/s/Mpc | Planck estimate inferred from CMB observations under a cosmological model. | Particle Data Group review, 2024. |
| 67.4 +4.1/−3.2 km/s/Mpc | Time-delay gravitational-lens estimate from 40 lenses; the uncertainty is asymmetric. | Birrer et al., as reported by the Particle Data Group, 2024. |
| 70 +12/−8 km/s/Mpc | Bright standard-siren estimate using the gravitational-wave event GW170817; the review describes the uncertainty as broad. | Particle Data Group review, 2024. |
The central comparison is the locally measured SH0ES value and the CMB-based Planck inference. The Particle Data Group’s 2024 review reports that these differ by about 5σ. NASA’s explainer gives approximate ranges of 70–76 km/s/Mpc for space-telescope measurements and 67–68 km/s/Mpc for CMB-derived values; these are rounded contextual ranges, not boundaries that apply to every method.
What Webb has—and has not—checked
Webb’s infrared observations help test whether crowding of stars or dust in Hubble images could have distorted Cepheid measurements. In a report dated 11 March 2024, ESA said Webb observations covered the Hubble measurement range and examined five supernova host galaxies containing eight Type Ia supernovae and roughly 1,000 Cepheids. The observations reached NGC 5468, about 130 million light-years away, and supported the reliability of Cepheid measurements across that range.
Rank #4
- Superior Optics: Dianfan astronomy telescope features a focal length of 800 mm and a fully coated 90 mm objective lens. The large aperture gathers more light for brighter, clearer views, high-transmission coatings enhance contrast and color for sharp, vivid images for an enjoyable stargazing experience
- Powerful Magnification: With two eyepieces and a 3× Barlow lens, this telescope offers 32×–240× magnification. It allows you to observe craters and lunar seas on the surface of the Moon. With the 45° zenith mirror, images are upright and correctly oriented for terrestrial viewing, making distant targets details easier to observe. The telescope is ideal for beginners and astronomy enthusiasts
- Quick Setup & Beginner-Friendly: User-friendly design with clear instructions makes assembly simple—ready in about 15 minutes. Smooth focusing helps you quickly locate targets. Perfect for families to enjoy stargazing and explore the night sky together
- Portable & Versatile: Lightweight design with a sturdy stainless steel tripod ensures stable, steady viewing. Easy to carry in the included bag, making it perfect for travel and outdoor adventures. Suitable for stargazing, birdwatching, wildlife observation, and exploring cityscapes
- Professional Astronomical Telescope: The Dianfan 90x800 telescope combines quality , functionality and reliability. It delivers high-resolution views that bring the universe closer. Designed for adults high powered, it inspires interest in astronomy and science while encouraging exploration of the starry sky
This is a cross-check of the Cepheid portion of the local distance ladder. It does not independently settle why the locally calibrated distance-ladder results differ from early-universe, CMB-based inferences. NASA and the Particle Data Group describe possible explanations—including underestimated systematic errors or changes to the cosmological model, such as new early-universe physics—as open possibilities, not established answers.
What other methods contribute
Researchers use other routes to check or extend the comparison. They do not all have the same precision, and none should be assumed free of calibration or model assumptions.
Best Value
- BRIGHT, SHARP VIEWS ANYWHERE: Unlike many beginner telescopes, this quality refractor features fully coated glass lenses and a 70mm aperture for crisp, clear views of the Moon, planets, and daytime scenery—all in a lightweight, travel-ready design.
- PERFECT FIRST TELESCOPE FOR BEGINNERS: Designed for adults and kids to enjoy together, this beginner-friendly telescope sets up in minutes and delivers rewarding views with no prior experience required.
- EASY NO-TOOL SETUP: No complicated assembly or tools needed. The full-height tripod and telescope tube set up in seconds and pack neatly into the included backpack—so you’re always ready to observe on the go.
- DUAL EYEPIECES FOR VERSATILE VIEWS: Includes 20mm and 10mm eyepieces for wide and close-up views of celestial and terrestrial targets. Spot craters on the Moon at night or wildlife during the day.
- BONUS BACKPACK AND SOFTWARE: Includes a padded backpack with room for accessories, a finderscope, star diagonal, eyepieces, and a free download of Starry Night software to help you identify celestial objects and plan your next stargazing session.
- Tip of the red-giant branch (TRGB): uses the characteristic brightness of red-giant stars to calibrate distances, including the CCHP Type Ia supernova route shown above.
- Baryon acoustic oscillations (BAO): use patterns in the distribution of matter. In an inverse distance ladder, BAO measurements help calibrate supernova distances; the inference depends on how the observables are connected.
- Strong-lens time delays: compare the arrival times of light that has travelled along different paths around a massive lensing object to constrain distances and the expansion rate.
- Gravitational-wave standard sirens: use gravitational-wave observations to estimate distance, then combine that with information about recession. The GW170817-based bright-siren estimate in the table has a broad uncertainty; the Particle Data Group says adding a small number of dark sirens still left results dominated by the bright event.
NASA LAMBDA’s method overview also plots approaches involving BAO plus baryon abundance, thermal Sunyaev–Zel’dovich measurements, and lensing. Their inclusion provides additional ways to compare results, not evidence that all methods currently agree or are equally precise.
How to compare an expansion-rate claim
Before treating two published numbers as competing measurements of the same kind, check what each one actually represents:
Quick Recap
- Early-universe inference or late-universe distance measurement? A CMB result uses a cosmological model to infer today’s rate; a local distance-ladder result builds outward from calibrated distances.
- Which observable and calibrator? Look for Cepheids, the red-giant-branch tip, supernovae, BAO, CMB patterns, lens time delays, or gravitational waves.
- Which data release and publication year? Measurements and analyses change; the 2024 Particle Data Group values above are a dated comparison, not a complete inventory of current results.
- What kind of uncertainty is quoted? Keep statistical and systematic uncertainties distinct when both are given, and preserve asymmetric ranges rather than reducing them to a single symmetric error.
- What assumptions connect the data to H0? This is especially important for CMB fits and BAO-based inverse ladders, where a model or calibration links the observed signal to today’s expansion rate.
Product prices and availability are accurate as of the date/time indicated and are subject to change. Any price and availability information displayed on Amazon at the time of purchase will apply.
