Poslinkio (Redshift) Apžvalgos ir Visatos Žemėlapiai

Shift (Redshift) Reviews and Universe Maps

Mapping millions of galaxies to understand large-scale structure, cosmic flux fields, and expansion

Why Shift Reviews Are Important

For centuries, astronomy has largely depicted celestial objects as points on a two-dimensional sphere. The third is distance – dimension remained elusive until the modern era. Hubble (Hubble) showed that the long-distance velocity (v) of galaxies is approximately proportional to their distance (d) (especially at small displacements), so galaxies redshift (shifts of spectral lines) has become a practical way to estimate cosmic distances. By systematically collecting large sets of galaxy shifts, three-dimensionally Maps of the structure of the universe – with threads, in clusters, voids and supercluster.

These large-scale reviews are now one of the essential observational cosmology pillars. They reveal space network, controlled dark matter and primary density fluctuations, and helps measure cosmic flows, the expansion history, the geometry and composition of the Universe. Below we discuss how displacement surveys work, what they have revealed, and how they help determine fundamental cosmological parameters (the fraction of dark energy, dark matter, the Hubble constant, etc.).


2. Fundamentals of Displacement and Cosmic Distances

2.1 Definition of Redshift

Galaxies redshift (z) is defined as follows:

z = (λobserved - λemitted) / λemitted, 

showing how much its spectral lines have shifted to longer wavelengths. For loved ones For galaxies, z ≈ v/c (v is the speed of motion, c is the speed of light). In more distant regions, cosmic expansion complicates the direct interpretation of the speed (v), but z remains a measure of how much the Universe has expanded since the moment of photon emission.

2.2 Hubble's Law and Larger Scales

At small redshifts (z ≪ 1) Hubble's law says: v ≈ H0 d. So, knowing the redshift, we can roughly determine the distance d ≈ (c/H0) z. For large z, a more detailed cosmological model (e.g., ΛCDM) relating z to the comoving distance. Thus, the essence of displacement surveys is to obtain redshift from spectral measurements (identification of spectral lines, e.g., hydrogen Balmer lines, [O II], etc.) and from this, distance, in order to create 3D maps of galaxies.


3. Overview of the Development of Shift Reviews

3.1 CfA Shift Overview

One of the early major reviews – Center for Astrophysics (CfA) Survey (1970s-1980s), which collected thousands of galaxy displacements. 2D wedge plots revealed "walls" and emptiness, including the "Great Wall". This showed that distribution of galaxies is far from homogeneous, and the large-scale structure extends over a scale of ~100 Mpc.

3.2 Two-Degree Field (2dF) and the Early 2000s

Early 2000s 2dF Galaxy Redshift Survey (2dFGRS), operating at the Anglo-Australian Telescope with a 2dF multi-aperture spectrograph, measured the displacements of ~220,000 galaxies up to z ∼ 0.3. This review confirmed baryonic acoustic oscillations (BAO) trace in the correlation function of galaxies, refined estimates of matter density, and made huge voids, thread and large-scale flow maps in unprecedented detail.

3.3 SDSS: Revolutionary Database

Started in 2000, Sloan Digital Sky Survey (SDSS) used a dedicated 2.5 m telescope with wide-field CCD imaging and multi-slit spectroscopy.Collected over several phases (SDSS-I, II, III, IV) millions spectra of galaxies, covering a large part of the northern sky. Sub-projects included:

  • BOSS (Baryon Oscillation Spectroscopic Survey): ~1.5 million red-luminous galaxies, allowing for extremely precise detection of BAOs.
  • eBOSS: Extended BAO studies to higher z using emission line galaxies, quasars, Lyα forest.
  • MY: Detailed integral field spectroscopy for thousands of galaxies.

The impact of SDSS is enormous: three-dimensional maps of the cosmic web, precise power spectra of galaxy clusters, and confirmation of the ΛCDM parameters with clear evidence of dark energy [1,2].

3.4 DESI, Euclid, Roman and the Future

DESI (Dark Energy Spectroscopic Instrument), which began operation in 2020, is aiming for ~35 million galaxy/quasar displacements up to z ∼ 3.5, further expanding the cosmic map. Upcoming projects:

  • Euclid (ESA) – wide-angle imaging and spectroscopy up to z ∼ 2.
  • Nancy Grace Roman Space Telescope (NASA) - will include near-IR observations, measure BAO and weak gravitational lensing.

Together with intensity mapping methods (e.g. for the SKA 21 cm line), these programs will allow large-scale structure to be studied at even higher redshifts, further refining the parameters of dark energy and the expansion history.


4. Large-Scale Structure: The Cosmic Network

4.1 Threads and Knots

Shift reviews show thread: elongated structures spanning tens or hundreds of Mpc and connecting dense "knots" or swarm. Found at the intersections of the filaments swarms, the densest environments of galaxies, while superclusters connect larger, more loosely coupled systems. Galaxies in filamentary zones may move along specific flow paths, supplementing the flow of material into the cluster centers.

4.2 Voids

Found among the threads emptiness – large, sparsely populated regions of matter that are almost entirely devoid of bright galaxies. They can be 10–50 Mpc in diameter or larger, occupying most of the universe but containing very few galaxies. Studies of voids help test dark energy, as the expansion in these sparser environments is somewhat faster, providing additional data on cosmic flows and gravity.

4.3 The whole

Filaments, clusters, superclusters, and voids together form network – the “foam-like” structure predicted in N-body simulations of dark matter. Observations confirm that dark matter is the main gravitational framework, and baryonic matter (stars, gas) only reflects this structure. It is displacement surveys that have allowed us to see the cosmic web both visually and quantitatively.


5. Cosmology from Shift Reviews

5.1 Correlation Function and Power Spectrum

One of the main tools is two-point correlation function ξ(r), which describes the excess probability of the distance r between a pair of galaxies compared to a random distribution. The power spectrum P(k) in Fourier space is also analyzed. The form of P(k) reveals the matter density, baryon fraction, neutrino mass, and the initial fluctuation spectrum. In combination with KFS data, the accuracy of the parameters fitted by ΛCDM increases significantly.

5.2 Baryonic Acoustic Oscillations (BAO)

The main feature of galaxy clusters is BAO signal, a weak peak in the correlation function at a scale of ~100–150 Mpc. This scale is well known from the physics of the early Universe, so it acts as "standard gauge" to measure cosmic distances by redshift.By comparing the measured BAO scale with a theoretical physical quantity, we obtain the Hubble parameter H(z). This helps constrain the equation of state of dark energy, cosmic geometry, and the evolution of the expansion of the Universe.

5.3 Regression Spatial Distortion (RSD)

Galaxies specific velocities along the visual lumen causes "redshift spatial distortions", disrupting the isotropy of the correlation function. From the RSD, one can conclude about growth rate of structures, so to check whether gravity is consistent with BR (general relativity) or whether there are any deviations. So far, the data agree with BR predictions, but new and future surveys will improve the accuracy, perhaps allowing us to detect slight deviations if new physics exists.


6. Cosmic Flow Maps

6.1 Movement of Specific Speed ​​and Local Group

In addition to Hubble's expansion, galaxies have specific speeds, arising from local accumulations of mass, e.g. Virginia Swarm, The Great Puller (Great Attractor). By combining displacements with independent distance indicators (Tully–Fisher method, supernovae, surface bright fluctuation methods) these velocity fields can be measured. "Cosmic flow" maps reveal velocity flows of hundreds of km/s on a scale of ~100 Mpc.

6.2 Discussions about the Common Flow

Some studies claim to have detected large-scale flows that exceed the ΛCDM expectation, but there are still significant systematic uncertainties. Detecting such cosmic flows provides additional insights into the distribution of dark matter or perhaps modified gravity. Combining displacement surveys with robust distance measurements further refines our maps of the velocity fields of the Universe.


7. Challenges and Systematic Biases

7.1 Selection Function and Completeness

Galaxies are often included in the displacement survey by magnitude-limited or color. Different sampling conditions or varying levels of detail across the sky can distort cluster measurements. Research groups carefully model the detail across different sky regions and adjust for radical selection (where the brightness decreases with distance, so fewer distant galaxies are captured). This ensures that the final correlation function or power spectrum is not artificially skewed.

7.2 Offset Errors and Photometric Methods

Spectroscopic the displacement can be accurate up to Δz ≈ 10-4. However, large photometric surveys (e.g., the Dark Energy Survey, LSST) use wide-band filters, so Δz is as high as 0.01–0.1. Although photometric surveys allow for the processing of a huge number of objects, the inaccuracies in the longitudinal direction (redshift direction) are larger. Such inaccuracies are mitigated by methods such as clustered offset calibration or cross-correlation with spectroscopic samples.

7.3 Nonlinear Evolution and the Prior Bias of Galaxies

At small scales, galaxy clusters become strongly nonlinear, due to finger-of-god effects in redshift space and complications caused by mergers. Galaxies also do not represent dark matter ideally - there is a "galaxy bias" factor that depends on the environment or the type of galaxy. Often, researchers use models or focus on larger scales (where the assumptions of linear theory hold) to reliably extract cosmological information.


8. Recent and Future Trends in Shift Reviews

8.1 DESI

Dark Energy Spectroscopic Instrument (DESI), mounted on the 4 m Mayall Telescope (Kitt Peak), began operation in 2020 and aims to measure 35 million spectra of galaxies and quasars. 5000 robotic fiber-optic trays allow thousands of displacements (z ∼ 0.05–3.5) to be obtained in just one exposure.This giant array will refine measurements of BAO distances over several cosmic epochs, determine features of expansion and growth of structures, and will also be invaluable for studies of galaxy evolution.

8.2 Euclid and Nancy Grace Roman Space Telescope

Euclid (ESA) and Roman (NASA) telescope, planned for the late 2020s, will combine near-IR imaging and spectroscopy, mapping billions of galaxies down to z ∼ 2. They will measure weak lensing and BAO, providing robust constraints on dark energy, possible cosmic curvature and neutrino mass. Collaboration with ground-based spectrographs and future intensity mapping systems (e.g. SKA 21 cm) will further expand the scope of the study.

8.3 21 cm Intensity Maps

New method – 21 cm intensity maps, where the brightness of the HI gas emission is measured over a large scale range, even without resolving individual galaxies. Arrays such as CHIME, HIRAX or SKA can detect BAO signatures in neutral hydrogen at even higher redshifts, reaching even the epochs of reionization. This is an additional way to constrain the expansion of the Universe, bypassing optical/IR shift survey methods, although calibration challenges remain.


9. Wider Influence: Dark Energy, the Hubble Tension, and More

9.1 Dark Energy Equation of State

By matching the BAO magnitude at various redshifts with the KFS data (z = 1100) and supernova data (at low z), we derive the expansion history H(z). This allows us to check whether dark energy is only cosmological constant (w = -1), or does it vary over time? So far, no clear difference from w = -1 has been found, but more accurate BAO data may reveal slight deviations.

9.2 Hubble Voltage

Some local ladder methods to obtain H0 The measurements exceed the ~67–68 km/s/Mpc determined by the Planck + BAO combination, the difference is 4–5σ. This "Hubble voltage" may be a sign of a systematic error or predict new physics (e.g. early dark energy). Further precise measurements of BAO (DESI, Euclid, etc.) will allow a better study of the intermediate redshifts, thus possibly resolving or increasing the tension.

9.3 Evolution of Galaxies

Displacement surveys also help studies of galaxy evolution: star formation history, morphological transformations, environmental influences. By comparing the properties of galaxies at different cosmic times, we learn how quenched galaxies, mergers, and gas inflows shape the overall population picture. The context of the cosmic web (filament or void) affects these processes, connecting the evolution of small-scale galaxies with the structure of large-scale galaxies.


10. Conclusion

Redshift overviews – essential observational cosmology tool that generates spatial maps of millions of galaxies. This 3D perspective reveals space network – filaments, clusters, voids – and allows for precise measurements of large-scale structure. Key achievements:

  • Baryonic acoustic oscillations (BAO): A standard gauge for cosmic distances that constrains dark energy.
  • Displacement spatial distortions: A study of the growth and gravity of structures.
  • Galactic flows and environment: Evolution of cosmic velocity fields and environmental effects.

Major surveys – from CfA to 2dF, SDSS, BOSS/eBOSS – have allowed the ΛCDM model to take hold, capturing a detailed picture of the cosmic web.Next generation projects – DESI, Euclid, Roman, 21 cm intensity maps – will further increase the redshift limits, further refining BAO distance values ​​and perhaps resolving the Hubble constant tension or opening up new physics. So the shift surveys remain precision cosmology at the forefront, showing how the large-scale structure of the Universe grows and how its expansion is governed by dark matter and dark energy.


Literature and Additional Reading

  1. de Lapparent, V., Geller, MJ, & Huchra, JP (1986). "A slice of the universe." The Astrophysical Journal Letters, 302, L1–L5.
  2. Eisenstein, DJ, et al. (2005). "Detection of the Baryon Acoustic Peak in the Large-Scale Correlation Function of SDSS Luminous Red Galaxies." The Astrophysical Journal, 633, 560–574.
  3. Cole, S., et al. (2005). "The 2dF Galaxy Redshift Survey: Power-spectrum analysis of the final data set and cosmological implications." Monthly Notices of the Royal Astronomical Society, 362, 505–534.
  4. Alam, S., et al. (2021). "Completed SDSS-IV extended Baryon Oscillation Spectroscopic Survey: Cosmological implications from two decades of spectroscopic surveys." Physical Review D, 103, 083533.
  5. DESI Collaboration: desi.lbl.gov (viewed 2023).
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